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fs: Add fchmodat2()

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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@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Alexey Gladkov <legion@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Message-Id: <f2a846ef495943c5d101011eebcf01179d0c7b61.1689092120.git.legion@kernel.org>
[brauner: pre reviews, do flag conversion in do_fchmodat() directly]
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Alexey Gladkov 2023-07-11 18:16:04 +02:00 • committed by Christian Brauner
parent 06a0213977
commit 09da082b07
No known key found for this signature in database
GPG key ID: 91C61BC06578DCA2
2 changed files with 19 additions and 4 deletions
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21
fs/open.c
View file

@ -671,11 +671,18 @@ 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,
unsigned int flags)
{
struct path path;
int error;
unsigned int lookup_flags = LOOKUP_FOLLOW;
unsigned int lookup_flags;
if (unlikely(flags & ~AT_SYMLINK_NOFOLLOW))
return -EINVAL;
lookup_flags = (flags & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW;
retry:
error = user_path_at(dfd, filename, lookup_flags, &path);
if (!error) {
@ -689,15 +696,21 @@ retry:
return error;
}
SYSCALL_DEFINE4(fchmodat2, int, dfd, const char __user *, filename,
umode_t, mode, unsigned int, flags)
{
return do_fchmodat(dfd, filename, mode, flags);
}
SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename,
umode_t, mode)
{
return do_fchmodat(dfd, filename, mode);
return do_fchmodat(dfd, filename, mode, 0);
}
SYSCALL_DEFINE2(chmod, const char __user *, filename, umode_t, mode)
{
return do_fchmodat(AT_FDCWD, filename, mode);
return do_fchmodat(AT_FDCWD, filename, mode, 0);
}
/*

2
include/linux/syscalls.h
View file

@ -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, unsigned 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);