Date: Wed, 22 Jan 2020 17:05:15 -0500 From: Rich Felker <dalias@...c.org> To: Paul Eggert <eggert@...ucla.edu> Cc: Florian Weimer <fweimer@...hat.com>, musl@...ts.openwall.com, 39236@...bugs.gnu.org Subject: Re: bug#39236: coreutils cp mishandles error return from lchmod On Wed, Jan 22, 2020 at 01:55:57PM -0800, Paul Eggert wrote: > On 1/22/20 7:08 AM, Florian Weimer wrote: > >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. > > That's too bad, because coreutils (and many other applications, I > expect) assume that lchmod (and fchmodat with AT_SYMLINK_NOFOLLOW) > to act like chmod except not follow symlinks, in order to make it > less likely that the application will run afoul of a symlink race > and chmod the wrong file. Isn't that how the Linux fstatat call > behaves? And if so, why does glibc fstatat refuse to support this > behavior? I think you're confusing fchmodat with fstatat. The Linux fchmodat syscall lacks a flags argument and thus doesn't suffice to implement fchmodat. The fstatat syscall does work. > To work around this bug, I suppose coreutils etc. should do > something like the following: > > 1. Never use lchmod since the porting nightmare is bad enough without it. > > 2. On non-glibc systems (or glibc systems where the bug is fixed), > use fchmodat with AT_SYMLINK_NOFOLLOW. > > 3. On glibc systems with the bug, use openat with > AT_SYMLINK_NOFOLLOW and O_PATH, and then fchmod the resulting file > descriptor. > > Does this sound right? Or is there some O_PATH gotcha that I haven't > thought about? I think fchmod historically did not work on O_PATH file descriptors, which is why musl is using chmod on the procfs magic symlink. However, fchmodat might work too with an empty pathname; I'm not sure. I think these fixes are better encapsulated as a replacement for missing/broken fchmodat, rather than putting the logic in individual utilities or coreutils-specific library code. Also, note that if you want to skip checking stat to make sure you didn't open a symlink with O_PATH, that depends on confirming Florian's claim that the kernel documents it will not follow the symlink. > Come to think of it, perhaps the best thing would be to change > Gnulib's lchmod and fchmodat modules so that they do what > applications expect, even on buggy glibc systems. (Which would be > ironic, since Gnulib's main goal is to put wrappers around other > libraries so that they look more like glibc.) I think we're approaching a consensus that glibc should fix this too, so then it would just be gnulib matching the fix. Rich
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