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Re: [PATCH 2/5] Add fchmodat4(), a new syscall




On Fri, 31 May 2019 12:51:00 PDT (-0700), Arnd Bergmann wrote:
On Fri, May 31, 2019 at 9:23 PM Palmer Dabbelt <palmer@xxxxxxxxxx> wrote:

man 3p says that fchmodat() takes a flags argument, but the Linux
syscall does not.  There doesn't appear to be a good userspace
workaround for this issue but the implementation in the kernel is pretty
straight-forward.  The specific use case where the missing flags came up
was WRT a fuse filesystem implemenation, but the functionality is pretty
generic so I'm assuming there would be other use cases.

Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxx>
---
 fs/open.c                | 21 +++++++++++++++++++--
 include/linux/syscalls.h |  5 +++++
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index a00350018a47..cfad7684e8d3 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -568,11 +568,17 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd, umode_t, mode)
        return ksys_fchmod(fd, mode);
 }

-int do_fchmodat(int dfd, const char __user *filename, umode_t mode)
+int do_fchmodat4(int dfd, const char __user *filename, umode_t mode, int flags)
...
+
+int do_fchmodat(int dfd, const char __user *filename, umode_t mode)
+{
+       return do_fchmodat4(dfd, filename, mode, 0);
+}
+

There is only one external caller of do_fchmodat(), so just change that
to pass the extra argument here, and keep a single do_fchmodat()
function used by the sys_fchmod(), sys_fchmod4(), sys_chmod()
and ksys_chmod().

OK, I'll roll that up into a v2.