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The documentation for seq_file suggests that it is necessary to be able to move the iterator to a given offset, however that is not the case. If the iterator is stored in the private data and is stable from one read() syscall to the next, it is only necessary to support first/next interactions. Implementing this in a client is a little clumsy. - if ->start() is given a pos of zero, it should go to start of sequence. - if ->start() is given the name pos that was given to the most recent next() or start(), it should restore the iterator to state just before that last call - if ->start is given another number, it should set the iterator one beyond the start just before the last ->start or ->next call. Also, the documentation says that the implementation can interpret the pos however it likes (other than zero meaning start), but seq_file increments the pos sometimes which does impose on the implementation. This patch simplifies the interface for first/next iteration and simplifies the code, while maintaining complete backward compatability. Now: - if ->start() is given a pos of zero, it should return an iterator placed at the start of the sequence - if ->start() is given a non-zero pos, it should return the iterator in the same state it was after the last ->start or ->next. This is particularly useful for interators which walk the multiple chains in a hash table, e.g. using rhashtable_walk*. See fs/gfs2/glock.c and drivers/staging/lustre/lustre/llite/vvp_dev.c A large part of achieving this is to *always* call ->next after ->show has successfully stored all of an entry in the buffer. Never just increment the index instead. Also: - always pass &m->index to ->start() and ->next(), never a temp variable - don't clear ->from when ->count is zero, as ->from is dead when ->count is zero. Some ->next functions do not increment *pos when they return NULL. To maintain compatability with this, we still need to increment m->index in one place, if ->next didn't increment it. Note that such ->next functions are buggy and should be fixed. A simple demonstration is dd if=/proc/swaps bs=1000 skip=1 Choose any block size larger than the size of /proc/swaps. This will always show the whole last line of /proc/swaps. This patch doesn't work around buggy next() functions for this case. [neilb@suse.com: ensure ->from is valid] Link: http://lkml.kernel.org/r/87601ryb8a.fsf@notabene.neil.brown.name Signed-off-by: NeilBrown <neilb@suse.com> Acked-by: Jonathan Corbet <corbet@lwn.net> [docs] Tested-by: Jann Horn <jannh@google.com> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Kees Cook <keescook@chromium.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
360 lines
14 KiB
Plaintext
360 lines
14 KiB
Plaintext
The seq_file interface
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Copyright 2003 Jonathan Corbet <corbet@lwn.net>
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This file is originally from the LWN.net Driver Porting series at
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http://lwn.net/Articles/driver-porting/
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There are numerous ways for a device driver (or other kernel component) to
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provide information to the user or system administrator. One useful
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technique is the creation of virtual files, in debugfs, /proc or elsewhere.
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Virtual files can provide human-readable output that is easy to get at
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without any special utility programs; they can also make life easier for
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script writers. It is not surprising that the use of virtual files has
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grown over the years.
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Creating those files correctly has always been a bit of a challenge,
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however. It is not that hard to make a virtual file which returns a
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string. But life gets trickier if the output is long - anything greater
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than an application is likely to read in a single operation. Handling
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multiple reads (and seeks) requires careful attention to the reader's
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position within the virtual file - that position is, likely as not, in the
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middle of a line of output. The kernel has traditionally had a number of
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implementations that got this wrong.
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The 2.6 kernel contains a set of functions (implemented by Alexander Viro)
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which are designed to make it easy for virtual file creators to get it
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right.
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The seq_file interface is available via <linux/seq_file.h>. There are
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three aspects to seq_file:
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* An iterator interface which lets a virtual file implementation
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step through the objects it is presenting.
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* Some utility functions for formatting objects for output without
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needing to worry about things like output buffers.
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* A set of canned file_operations which implement most operations on
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the virtual file.
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We'll look at the seq_file interface via an extremely simple example: a
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loadable module which creates a file called /proc/sequence. The file, when
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read, simply produces a set of increasing integer values, one per line. The
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sequence will continue until the user loses patience and finds something
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better to do. The file is seekable, in that one can do something like the
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following:
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dd if=/proc/sequence of=out1 count=1
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dd if=/proc/sequence skip=1 of=out2 count=1
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Then concatenate the output files out1 and out2 and get the right
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result. Yes, it is a thoroughly useless module, but the point is to show
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how the mechanism works without getting lost in other details. (Those
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wanting to see the full source for this module can find it at
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http://lwn.net/Articles/22359/).
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Deprecated create_proc_entry
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Note that the above article uses create_proc_entry which was removed in
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kernel 3.10. Current versions require the following update
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- entry = create_proc_entry("sequence", 0, NULL);
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- if (entry)
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- entry->proc_fops = &ct_file_ops;
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+ entry = proc_create("sequence", 0, NULL, &ct_file_ops);
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The iterator interface
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Modules implementing a virtual file with seq_file must implement an
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iterator object that allows stepping through the data of interest
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during a "session" (roughly one read() system call). If the iterator
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is able to move to a specific position - like the file they implement,
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though with freedom to map the position number to a sequence location
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in whatever way is convenient - the iterator need only exist
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transiently during a session. If the iterator cannot easily find a
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numerical position but works well with a first/next interface, the
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iterator can be stored in the private data area and continue from one
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session to the next.
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A seq_file implementation that is formatting firewall rules from a
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table, for example, could provide a simple iterator that interprets
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position N as the Nth rule in the chain. A seq_file implementation
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that presents the content of a, potentially volatile, linked list
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might record a pointer into that list, providing that can be done
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without risk of the current location being removed.
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Positioning can thus be done in whatever way makes the most sense for
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the generator of the data, which need not be aware of how a position
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translates to an offset in the virtual file. The one obvious exception
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is that a position of zero should indicate the beginning of the file.
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The /proc/sequence iterator just uses the count of the next number it
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will output as its position.
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Four functions must be implemented to make the iterator work. The
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first, called start(), starts a session and takes a position as an
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argument, returning an iterator which will start reading at that
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position. The pos passed to start() will always be either zero, or
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the most recent pos used in the previous session.
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For our simple sequence example,
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the start() function looks like:
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static void *ct_seq_start(struct seq_file *s, loff_t *pos)
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{
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loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL);
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if (! spos)
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return NULL;
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*spos = *pos;
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return spos;
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}
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The entire data structure for this iterator is a single loff_t value
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holding the current position. There is no upper bound for the sequence
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iterator, but that will not be the case for most other seq_file
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implementations; in most cases the start() function should check for a
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"past end of file" condition and return NULL if need be.
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For more complicated applications, the private field of the seq_file
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structure can be used to hold state from session to session. There is
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also a special value which can be returned by the start() function
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called SEQ_START_TOKEN; it can be used if you wish to instruct your
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show() function (described below) to print a header at the top of the
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output. SEQ_START_TOKEN should only be used if the offset is zero,
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however.
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The next function to implement is called, amazingly, next(); its job is to
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move the iterator forward to the next position in the sequence. The
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example module can simply increment the position by one; more useful
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modules will do what is needed to step through some data structure. The
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next() function returns a new iterator, or NULL if the sequence is
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complete. Here's the example version:
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static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos)
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{
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loff_t *spos = v;
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*pos = ++*spos;
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return spos;
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}
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The stop() function closes a session; its job, of course, is to clean
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up. If dynamic memory is allocated for the iterator, stop() is the
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place to free it; if a lock was taken by start(), stop() must release
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that lock. The value that *pos was set to by the last next() call
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before stop() is remembered, and used for the first start() call of
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the next session unless lseek() has been called on the file; in that
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case next start() will be asked to start at position zero.
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static void ct_seq_stop(struct seq_file *s, void *v)
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{
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kfree(v);
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}
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Finally, the show() function should format the object currently pointed to
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by the iterator for output. The example module's show() function is:
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static int ct_seq_show(struct seq_file *s, void *v)
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{
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loff_t *spos = v;
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seq_printf(s, "%lld\n", (long long)*spos);
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return 0;
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}
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If all is well, the show() function should return zero. A negative error
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code in the usual manner indicates that something went wrong; it will be
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passed back to user space. This function can also return SEQ_SKIP, which
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causes the current item to be skipped; if the show() function has already
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generated output before returning SEQ_SKIP, that output will be dropped.
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We will look at seq_printf() in a moment. But first, the definition of the
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seq_file iterator is finished by creating a seq_operations structure with
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the four functions we have just defined:
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static const struct seq_operations ct_seq_ops = {
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.start = ct_seq_start,
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.next = ct_seq_next,
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.stop = ct_seq_stop,
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.show = ct_seq_show
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};
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This structure will be needed to tie our iterator to the /proc file in
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a little bit.
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It's worth noting that the iterator value returned by start() and
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manipulated by the other functions is considered to be completely opaque by
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the seq_file code. It can thus be anything that is useful in stepping
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through the data to be output. Counters can be useful, but it could also be
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a direct pointer into an array or linked list. Anything goes, as long as
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the programmer is aware that things can happen between calls to the
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iterator function. However, the seq_file code (by design) will not sleep
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between the calls to start() and stop(), so holding a lock during that time
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is a reasonable thing to do. The seq_file code will also avoid taking any
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other locks while the iterator is active.
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Formatted output
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The seq_file code manages positioning within the output created by the
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iterator and getting it into the user's buffer. But, for that to work, that
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output must be passed to the seq_file code. Some utility functions have
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been defined which make this task easy.
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Most code will simply use seq_printf(), which works pretty much like
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printk(), but which requires the seq_file pointer as an argument.
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For straight character output, the following functions may be used:
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seq_putc(struct seq_file *m, char c);
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seq_puts(struct seq_file *m, const char *s);
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seq_escape(struct seq_file *m, const char *s, const char *esc);
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The first two output a single character and a string, just like one would
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expect. seq_escape() is like seq_puts(), except that any character in s
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which is in the string esc will be represented in octal form in the output.
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There are also a pair of functions for printing filenames:
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int seq_path(struct seq_file *m, const struct path *path,
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const char *esc);
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int seq_path_root(struct seq_file *m, const struct path *path,
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const struct path *root, const char *esc)
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Here, path indicates the file of interest, and esc is a set of characters
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which should be escaped in the output. A call to seq_path() will output
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the path relative to the current process's filesystem root. If a different
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root is desired, it can be used with seq_path_root(). If it turns out that
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path cannot be reached from root, seq_path_root() returns SEQ_SKIP.
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A function producing complicated output may want to check
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bool seq_has_overflowed(struct seq_file *m);
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and avoid further seq_<output> calls if true is returned.
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A true return from seq_has_overflowed means that the seq_file buffer will
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be discarded and the seq_show function will attempt to allocate a larger
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buffer and retry printing.
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Making it all work
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So far, we have a nice set of functions which can produce output within the
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seq_file system, but we have not yet turned them into a file that a user
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can see. Creating a file within the kernel requires, of course, the
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creation of a set of file_operations which implement the operations on that
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file. The seq_file interface provides a set of canned operations which do
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most of the work. The virtual file author still must implement the open()
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method, however, to hook everything up. The open function is often a single
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line, as in the example module:
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static int ct_open(struct inode *inode, struct file *file)
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{
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return seq_open(file, &ct_seq_ops);
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}
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Here, the call to seq_open() takes the seq_operations structure we created
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before, and gets set up to iterate through the virtual file.
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On a successful open, seq_open() stores the struct seq_file pointer in
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file->private_data. If you have an application where the same iterator can
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be used for more than one file, you can store an arbitrary pointer in the
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private field of the seq_file structure; that value can then be retrieved
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by the iterator functions.
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There is also a wrapper function to seq_open() called seq_open_private(). It
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kmallocs a zero filled block of memory and stores a pointer to it in the
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private field of the seq_file structure, returning 0 on success. The
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block size is specified in a third parameter to the function, e.g.:
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static int ct_open(struct inode *inode, struct file *file)
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{
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return seq_open_private(file, &ct_seq_ops,
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sizeof(struct mystruct));
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}
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There is also a variant function, __seq_open_private(), which is functionally
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identical except that, if successful, it returns the pointer to the allocated
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memory block, allowing further initialisation e.g.:
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static int ct_open(struct inode *inode, struct file *file)
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{
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struct mystruct *p =
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__seq_open_private(file, &ct_seq_ops, sizeof(*p));
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if (!p)
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return -ENOMEM;
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p->foo = bar; /* initialize my stuff */
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...
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p->baz = true;
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return 0;
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}
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A corresponding close function, seq_release_private() is available which
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frees the memory allocated in the corresponding open.
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The other operations of interest - read(), llseek(), and release() - are
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all implemented by the seq_file code itself. So a virtual file's
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file_operations structure will look like:
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static const struct file_operations ct_file_ops = {
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.owner = THIS_MODULE,
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.open = ct_open,
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.read = seq_read,
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.llseek = seq_lseek,
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.release = seq_release
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};
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There is also a seq_release_private() which passes the contents of the
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seq_file private field to kfree() before releasing the structure.
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The final step is the creation of the /proc file itself. In the example
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code, that is done in the initialization code in the usual way:
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static int ct_init(void)
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{
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struct proc_dir_entry *entry;
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proc_create("sequence", 0, NULL, &ct_file_ops);
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return 0;
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}
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module_init(ct_init);
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And that is pretty much it.
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seq_list
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If your file will be iterating through a linked list, you may find these
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routines useful:
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struct list_head *seq_list_start(struct list_head *head,
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loff_t pos);
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struct list_head *seq_list_start_head(struct list_head *head,
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loff_t pos);
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struct list_head *seq_list_next(void *v, struct list_head *head,
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loff_t *ppos);
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These helpers will interpret pos as a position within the list and iterate
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accordingly. Your start() and next() functions need only invoke the
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seq_list_* helpers with a pointer to the appropriate list_head structure.
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The extra-simple version
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For extremely simple virtual files, there is an even easier interface. A
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module can define only the show() function, which should create all the
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output that the virtual file will contain. The file's open() method then
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calls:
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int single_open(struct file *file,
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int (*show)(struct seq_file *m, void *p),
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void *data);
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When output time comes, the show() function will be called once. The data
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value given to single_open() can be found in the private field of the
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seq_file structure. When using single_open(), the programmer should use
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single_release() instead of seq_release() in the file_operations structure
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to avoid a memory leak.
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