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The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:
struct foo {
int stuff;
struct boo array[];
};
By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.
Also, notice that, dynamic memory allocations won't be affected by
this change:
"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]
sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.
Also, the following issue shows up due to the flexible-array member
having incomplete type[4]:
drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_bios_measurements_start’:
drivers/char/tpm/eventlog/tpm2.c:54:46: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’}
54 | size = sizeof(struct tcg_pcr_event) - sizeof(event_header->event)
| ^
drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_bios_measurements_next’:
drivers/char/tpm/eventlog/tpm2.c:102:10: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’}
102 | sizeof(event_header->event) + event_header->event_size;
| ^
drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_binary_bios_measurements_show’:
drivers/char/tpm/eventlog/tpm2.c:140:10: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’}
140 | sizeof(event_header->event) + event_header->event_size;
| ^
scripts/Makefile.build:266: recipe for target 'drivers/char/tpm/eventlog/tpm2.o' failed
make[3]: *** [drivers/char/tpm/eventlog/tpm2.o] Error 1
As mentioned above: "Flexible array members have incomplete type, and
so the sizeof operator may not be applied. As a quirk of the original
implementation of zero-length arrays, sizeof evaluates to zero."[1] As
in "sizeof(event_header->event) always evaluated to 0, so removing it
has no effect".
Lastly, make use of the struct_size() helper to deal with the
flexible array member and its host structure.
This issue was found with the help of Coccinelle.
[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit
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| .. | ||
| agp | ||
| hw_random | ||
| ipmi | ||
| mwave | ||
| pcmcia | ||
| tpm | ||
| xilinx_hwicap | ||
| xillybus | ||
| adi.c | ||
| apm-emulation.c | ||
| applicom.c | ||
| applicom.h | ||
| bsr.c | ||
| ds1620.c | ||
| dsp56k.c | ||
| dtlk.c | ||
| hangcheck-timer.c | ||
| hpet.c | ||
| Kconfig | ||
| lp.c | ||
| Makefile | ||
| mem.c | ||
| misc.c | ||
| mspec.c | ||
| nsc_gpio.c | ||
| nvram.c | ||
| nwbutton.c | ||
| nwbutton.h | ||
| nwflash.c | ||
| pc8736x_gpio.c | ||
| powernv-op-panel.c | ||
| ppdev.c | ||
| ps3flash.c | ||
| random.c | ||
| raw.c | ||
| scx200_gpio.c | ||
| sonypi.c | ||
| tb0219.c | ||
| tlclk.c | ||
| toshiba.c | ||
| ttyprintk.c | ||
| uv_mmtimer.c | ||
| virtio_console.c | ||