On Mon, Aug 26, 2024 at 3:57 PM Alejandro Colomar <alx@xxxxxxxxxx> wrote: > > 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? what's your timeline for "currently interesting" vs "just historical"? (though i guess whatever that is, Linux 6.6 is pretty new at less than 12 months old!) > $ 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? if you just want the trivial removal of the incorrect sentence, sure ... if you want a whole new "C library/kernel differences" section you're probably able to write that ... which is probably a good juncture to point out that there's also a whole missing function --- there's no mention of lchmod() which seems to have been added in GLIBC_2.3.2 (yes, 22 years ago; not to be confused with 2.32 :-) ... except i don't think it _worked_ on Linux until glibc sha 6b89c385d8bd0700b25bac2c2d0bebe68d5cc05d in 2020? i don't know how you want to document that!). > 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/>
