path: root/src/stat
diff options
authorRich Felker <>2013-08-02 12:25:32 -0400
committerRich Felker <>2013-08-02 12:25:32 -0400
commit0dc4824479e357a3e23a02d35527e23fca920343 (patch)
treea293864b7e9dc56a3ed9488576134a68812b82b1 /src/stat
parent3e3753c1a8e047dc84f9db1dc26bb046cff457a6 (diff)
work around linux's lack of flags argument to fchmodat syscall
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.
Diffstat (limited to 'src/stat')
1 files changed, 29 insertions, 1 deletions
diff --git a/src/stat/fchmodat.c b/src/stat/fchmodat.c
index 61d32065..c1410bc5 100644
--- a/src/stat/fchmodat.c
+++ b/src/stat/fchmodat.c
@@ -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);