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Subject[PATCH v3 2/5] fs: Add fchmodat4()
Date
From: Palmer Dabbelt <palmer@sifive.com>

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 fchmodat4() 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

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Alexey Gladkov <legion@kernel.org>
---
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 4478adcc4f3a..58bb88c6afb6 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_fchmodat4(int dfd, const char __user *filename, umode_t mode, int lookup_flags)
{
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(fchmodat4, int, dfd, const char __user *, filename,
+ umode_t, mode, int, flags)
+{
+ if (unlikely(flags & ~AT_SYMLINK_NOFOLLOW))
+ return -EINVAL;
+
+ return do_fchmodat4(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_fchmodat4(dfd, filename, mode, LOOKUP_FOLLOW);
}

SYSCALL_DEFINE2(chmod, const char __user *, filename, umode_t, mode)
{
- return do_fchmodat(AT_FDCWD, filename, mode);
+ return do_fchmodat4(AT_FDCWD, filename, mode, LOOKUP_FOLLOW);
}

/**
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 497bdd968c32..b17d37d2bad6 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -466,6 +466,8 @@ 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_fchmodat4(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
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