On 2023-07-11, Alexey Gladkov <legion@xxxxxxxxxx> wrote: > On the userspace side fchmodat(3) is implemented as a wrapper > function which implements the POSIX-specified interface. This > interface differs from the underlying kernel system call, which does not > have a flags argument. Most implementations require procfs [1][2]. > > There doesn't appear to be a good userspace workaround for this issue > but the implementation in the kernel is pretty straight-forward. > > The new fchmodat2() syscall allows to pass the AT_SYMLINK_NOFOLLOW flag, > unlike existing fchmodat. > > [1] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/fchmodat.c;h=17eca54051ee28ba1ec3f9aed170a62630959143;hb=a492b1e5ef7ab50c6fdd4e4e9879ea5569ab0a6c#l35 > [2] https://git.musl-libc.org/cgit/musl/tree/src/stat/fchmodat.c?id=718f363bc2067b6487900eddc9180c84e7739f80#n28 > > Co-developed-by: Palmer Dabbelt <palmer@xxxxxxxxxx> > Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxx> > Signed-off-by: Alexey Gladkov <legion@xxxxxxxxxx> > Acked-by: Arnd Bergmann <arnd@xxxxxxxx> > --- > fs/open.c | 18 ++++++++++++++---- > include/linux/syscalls.h | 2 ++ > 2 files changed, 16 insertions(+), 4 deletions(-) > > diff --git a/fs/open.c b/fs/open.c > index 0c55c8e7f837..39a7939f0d00 100644 > --- a/fs/open.c > +++ b/fs/open.c > @@ -671,11 +671,11 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd, umode_t, mode) > return err; > } > > -static int do_fchmodat(int dfd, const char __user *filename, umode_t mode) > +static int do_fchmodat(int dfd, const char __user *filename, umode_t mode, int lookup_flags) I think it'd be much neater to do the conversion of AT_ flags here and pass 0 as a flags argument for all of the wrappers (this is how most of the other xyz(), fxyz(), fxyzat() syscall wrappers are done IIRC). > { > struct path path; > int error; > - unsigned int lookup_flags = LOOKUP_FOLLOW; > + > retry: > error = user_path_at(dfd, filename, lookup_flags, &path); > if (!error) { > @@ -689,15 +689,25 @@ static int do_fchmodat(int dfd, const char __user *filename, umode_t mode) > return error; > } > > +SYSCALL_DEFINE4(fchmodat2, int, dfd, const char __user *, filename, > + umode_t, mode, int, flags) > +{ > + if (unlikely(flags & ~AT_SYMLINK_NOFOLLOW)) > + return -EINVAL; We almost certainly want to support AT_EMPTY_PATH at the same time. Otherwise userspace will still need to go through /proc when trying to chmod a file handle they have. > + > + return do_fchmodat(dfd, filename, mode, > + flags & AT_SYMLINK_NOFOLLOW ? 0 : LOOKUP_FOLLOW); > +} > + > SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename, > umode_t, mode) > { > - return do_fchmodat(dfd, filename, mode); > + return do_fchmodat(dfd, filename, mode, LOOKUP_FOLLOW); > } > > SYSCALL_DEFINE2(chmod, const char __user *, filename, umode_t, mode) > { > - return do_fchmodat(AT_FDCWD, filename, mode); > + return do_fchmodat(AT_FDCWD, filename, mode, LOOKUP_FOLLOW); > } > > /* > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h > index 584f404bf868..6e852279fbc3 100644 > --- a/include/linux/syscalls.h > +++ b/include/linux/syscalls.h > @@ -440,6 +440,8 @@ asmlinkage long sys_chroot(const char __user *filename); > asmlinkage long sys_fchmod(unsigned int fd, umode_t mode); > asmlinkage long sys_fchmodat(int dfd, const char __user *filename, > umode_t mode); > +asmlinkage long sys_fchmodat2(int dfd, const char __user *filename, > + umode_t mode, int flags); > asmlinkage long sys_fchownat(int dfd, const char __user *filename, uid_t user, > gid_t group, int flag); > asmlinkage long sys_fchown(unsigned int fd, uid_t user, gid_t group); > -- > 2.33.8 > -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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