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work around linux's lack of flags argument to fchmodat syscall

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previously, the AT_SYMLINK_NOFOLLOW flag was ignored, giving
dangerously incorrect behavior -- the target of the symlink had its
modes changed to the modes (usually 0777) intended for the symlink).
this issue was amplified by the fact that musl provides lchmod, as a
wrapper for fchmodat, which some archival programs take as a sign that
symlink modes are supported and thus attempt to use.

emulating AT_SYMLINK_NOFOLLOW was a difficult problem, and I
originally believed it could not be solved, at least not without
depending on kernels newer than 3.5.x or so where O_PATH works halfway
well. however, it turns out that accessing O_PATH file descriptors via
their pseudo-symlink entries in /proc/self/fd works much better than
trying to use the fd directly, and works even on older kernels.
moreover, the kernel has permanently pegged these references to the
inode obtained by the O_PATH open, so there should not be race
conditions with the file being moved, deleted, replaced, etc.
This commit is contained in:
Rich Felker 2013-08-02 12:25:32 -04:00
parent 3e3753c1a8
commit 0dc4824479
1 changed files with 29 additions and 1 deletions
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30
src/stat/fchmodat.c
View File

@ -1,7 +1,35 @@
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
#include "syscall.h"
int fchmodat(int fd, const char *path, mode_t mode, int flag)
{
return syscall(SYS_fchmodat, fd, path, mode, flag);
if (!flag) return syscall(SYS_fchmodat, fd, path, mode, flag);
if (flag != AT_SYMLINK_NOFOLLOW)
return __syscall_ret(-EINVAL);
struct stat st;
int ret, fd2;
char proc[15+3*sizeof(int)];
if ((ret = __syscall(SYS_fstatat, fd, path, &st, flag)))
return __syscall_ret(ret);
if (S_ISLNK(st.st_mode))
return __syscall_ret(-EOPNOTSUPP);
if ((fd2 = __syscall(SYS_openat, fd, path, O_RDONLY|O_PATH|O_NOFOLLOW|O_NOCTTY)) < 0) {
if (fd2 == -ELOOP)
return __syscall_ret(-EOPNOTSUPP);
return __syscall_ret(fd2);
}
snprintf(proc, sizeof proc, "/proc/self/fd/%d", fd2);
if (!(ret = __syscall(SYS_stat, proc, &st)) && !S_ISLNK(st.st_mode))
ret = __syscall(SYS_chmod, proc, mode);
__syscall(SYS_close, fd2);
return __syscall_ret(ret);
}