Date: Wed, 22 Jan 2020 17:19:05 +0100 From: Florian Weimer <fweimer@...hat.com> To: Rich Felker <dalias@...c.org> Cc: 39236@...bugs.gnu.org, musl@...ts.openwall.com Subject: Re: coreutils cp mishandles error return from lchmod * Rich Felker: > On Wed, Jan 22, 2020 at 04:32:45PM +0100, Florian Weimer wrote: >> * Rich Felker: >> >> > On Wed, Jan 22, 2020 at 04:08:26PM +0100, Florian Weimer wrote: >> >> * Rich Felker: >> >> >> >> > On Wed, Jan 22, 2020 at 03:34:18PM +0100, Florian Weimer wrote: >> >> >> * Rich Felker: >> >> >> >> >> >> > coreutils should be opting to use the system-provided lchmod, which is >> >> >> > safe, and correctly handling error returns (silently treating >> >> >> > EOPNOTSUPP as success) rather than as hard errors. >> >> >> >> >> >> glibc's lchmod always returns ENOSYS (except on Hurd). I don't know how >> >> >> lchmod is used in coreutils, but I suspect it is not particularly >> >> >> useful. >> >> > >> >> > When preserving permissions (cp -p, archive extraction, etc.), you >> >> > want lchmod to work correctly just for the purpose of *not* following >> >> > the link and thereby unwantedly changing the permissions of the link >> >> > target. But, fchmodat with AT_SYMLINK_NOFOLLOW works just as well and >> >> > is standard, and that's really what coreutils should be using. >> >> >> >> I think you misread what I wrote: lchmod *always* returns ENOSYS. Even >> >> if the file is not a symbolic link. Likewise, fchmodat with >> >> AT_SYMLINK_NOFOLLOW *always* returns ENOTSUP. >> > >> > Yes, I understood that. I was going into why there should be a real >> > implementation, but didn't make it clear that that was what I was >> > doing. >> >> Ah, yes, there should be a real implementation if we can get full >> lchmod/AT_SYMLINK_NOFOLLOW behavior on file systems that support it. If >> we can't, I'm not sure if there is a point to it. > > The point is to fail when the target is a symlink, rather than > (erroneously and possibly dangerously) applying the chmod to the link > target. Actually supporting link modes is useless. It's the "not > modifying the target" that's important. The kernel supports it on some file systems, though: $ ls -l /tmp/x l---------. 1 fweimer fweimer 6 Jan 22 15:27 /tmp/x -> /tmp/x Although mode 0 curiously does not prevent readlink calls. > It's explained in the bz you just replied on, > https://sourceware.org/bugzilla/show_bug.cgi?id=14578 > > The point of the S_ISLNK check is to fail out early with the ENOTSUPP, > which the caller should treat as "success-like", in the non-racing > condition, without the need to open a fd (which may fail with > ENFILE/EMFILE) and without the need for /proc to be mounted. > Otherwise, a different error will be produced when one of those cases > is hit, and the caller will treat it as a real error. Hmm. The way I read the musl code, the O_PATH descriptor already exists. At this point, you can just chmod the O_PATH descriptor, and have the kernel report EOPNOTSUPP if the file system does not support that. Thanks, Florian
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