Hi Elliott, On Mon, Aug 26, 2024 at 12:37:46PM GMT, enh wrote: > looks like Linux 6.6... Thanks for researching that! > commit 475d4df82719225510625b4263baa1105665f4b3 > Merge: 511fb5bafed1 712143795327 > Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Date: Mon Aug 28 11:25:27 2023 -0700 > > Merge tag 'v6.6-vfs.fchmodat2' of > git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs > > Pull fchmodat2 system call from Christian Brauner: > "This adds the fchmodat2() system call. It is a revised version of the > fchmodat() system call, adding a missing flag argument. Support for > both AT_SYMLINK_NOFOLLOW and AT_EMPTY_PATH are included. Maybe we need to add a "C library/kernel differences" section? $ grep -rh '^\.SS ' man | sort | uniq -c | sort | tail 5 .SS glibc 7 .SS Authors and copyright conditions 8 .SS Address format 11 .SS Ioctls 11 .SS glibc notes 12 .SS Socket options 13 .SS /proc interfaces 15 .SS ISO/IEC\~8859 alphabets 51 .SS C library/kernel differences 99 .SS Program source Would you mind preparing a patch? Have a lovely day! Alex > Adding this system call revision has been a longstanding request but > so far has always fallen through the cracks. While the kernel > implementation of fchmodat() does not have a flag argument the libc > provided POSIX-compliant fchmodat(3) version does. Both glibc and musl > have to implement a workaround in order to support AT_SYMLINK_NOFOLLOW > (see [1] and [2]). > > The workaround is brittle because it relies not just on O_PATH and > O_NOFOLLOW semantics and procfs magic links but also on our rather > inconsistent symlink semantics. > > This gives userspace a proper fchmodat2() system call that libcs can > use to properly implement fchmodat(3) and allows them to get rid of > their hacks. In this case it will immediately benefit them as the > current workaround is already defunct because of aformentioned > inconsistencies. > > In addition to AT_SYMLINK_NOFOLLOW, give userspace the ability to use > AT_EMPTY_PATH with fchmodat2(). This is already possible with > fchownat() so there's no reason to not also support it for > fchmodat2(). > > The implementation is simple and comes with selftests. Implementation > of the system call and wiring up the system call are done as separate > patches even though they could arguably be one patch. But in case > there are merge conflicts from other system call additions it can be > beneficial to have separate patches" > > Link: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/fchmodat.c;h=17eca54051ee28ba1ec3f9aed170a62630959143;hb=a492b1e5ef7ab50c6fdd4e4e9879ea5569ab0a6c#l35 > [1] > Link: https://git.musl-libc.org/cgit/musl/tree/src/stat/fchmodat.c?id=718f363bc2067b6487900eddc9180c84e7739f80#n28 > [2] > > * tag 'v6.6-vfs.fchmodat2' of > git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs: > selftests: fchmodat2: remove duplicate unneeded defines > fchmodat2: add support for AT_EMPTY_PATH > selftests: Add fchmodat2 selftest > arch: Register fchmodat2, usually as syscall 452 > fs: Add fchmodat2() > Non-functional cleanup of a "__user * filename" > > On Sun, Aug 25, 2024 at 5:52 AM Alejandro Colomar <alx@xxxxxxxxxx> wrote: > > > > Hi Elliott, > > > > On Fri, Aug 23, 2024 at 02:40:16PM GMT, enh wrote: > > > subject says it all... the "This flag is not currently implemented." > > > on the man page is wrong, i think? > > > > I guess it was right when the text was written in 2006. > > > > $ git blame --follow -- man/man2/chmod.2 | grep currently > > 92f114db82 man2/chmod.2 (Michael Kerrisk 2014-02-21 08:35:27 +0100 207) This flag is not currently implemented. > > $ git blame 92f114db82^ -- man2/fchmodat.2 | grep currently > > a53b8cb2a0 (Michael Kerrisk 2006-05-02 00:05:06 +0000 99) This flag is not currently implemented. > > > > That might perfectly have changed in the last 18 years. :) > > > > Would you mind writing a small program and shell session that > > demonstrates it? It could be interesting for an EXAMPLES section. > > > > > > Have a lovely day! > > Alex > > > > -- > > <https://www.alejandro-colomar.es/> -- <https://www.alejandro-colomar.es/>
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