From 0dc4824479e357a3e23a02d35527e23fca920343 Mon Sep 17 00:00:00 2001 From: Rich Felker Date: Fri, 2 Aug 2013 12:25:32 -0400 Subject: 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. --- src/stat/fchmodat.c | 30 +++++++++++++++++++++++++++++- 1 file changed, 29 insertions(+), 1 deletion(-) (limited to 'src/stat/fchmodat.c') 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 +#include +#include +#include #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); } -- cgit v1.2.1