diff mbox series

[v2,2/2] vfs: add fchmodat2 syscall

Message ID 20200916002335.GQ3265@brightrain.aerifal.cx
State New
Headers show
Series changes for addding fchmodat2 syscall | expand

Commit Message

Rich Felker Sept. 16, 2020, 12:23 a.m. UTC
POSIX defines fchmodat as having a 4th argument, flags, that can be
AT_SYMLINK_NOFOLLOW. Support for changing the access mode of symbolic
links is optional (EOPNOTSUPP allowed if not supported), but this flag
is important even on systems where symlinks do not have access modes,
since it's the only way to safely change the mode of a file which
might be asynchronously replaced with a symbolic link, without a race
condition whereby the link target is changed.

It's possible to emulate AT_SYMLINK_NOFOLLOW in userspace, and both
musl libc and glibc do this, by opening an O_PATH file descriptor and
performing chmod on the corresponding magic symlink in /proc/self/fd.
However, this requires procfs to be mounted and accessible.

Signed-off-by: Rich Felker <dalias@libc.org>
---
 arch/alpha/kernel/syscalls/syscall.tbl      |  1 +
 arch/arm/tools/syscall.tbl                  |  1 +
 arch/arm64/include/asm/unistd.h             |  2 +-
 arch/arm64/include/asm/unistd32.h           |  2 ++
 arch/ia64/kernel/syscalls/syscall.tbl       |  1 +
 arch/m68k/kernel/syscalls/syscall.tbl       |  1 +
 arch/microblaze/kernel/syscalls/syscall.tbl |  1 +
 arch/mips/kernel/syscalls/syscall_n32.tbl   |  1 +
 arch/mips/kernel/syscalls/syscall_n64.tbl   |  1 +
 arch/mips/kernel/syscalls/syscall_o32.tbl   |  1 +
 arch/parisc/kernel/syscalls/syscall.tbl     |  1 +
 arch/powerpc/kernel/syscalls/syscall.tbl    |  1 +
 arch/s390/kernel/syscalls/syscall.tbl       |  1 +
 arch/sh/kernel/syscalls/syscall.tbl         |  1 +
 arch/sparc/kernel/syscalls/syscall.tbl      |  1 +
 arch/x86/entry/syscalls/syscall_32.tbl      |  1 +
 arch/x86/entry/syscalls/syscall_64.tbl      |  1 +
 arch/xtensa/kernel/syscalls/syscall.tbl     |  1 +
 fs/open.c                                   | 17 ++++++++++++++---
 include/linux/syscalls.h                    |  2 ++
 include/uapi/asm-generic/unistd.h           |  4 +++-
 21 files changed, 38 insertions(+), 5 deletions(-)

Comments

Aleksa Sarai Sept. 16, 2020, 6:01 a.m. UTC | #1
On 2020-09-15, Rich Felker <dalias@libc.org> wrote:
> POSIX defines fchmodat as having a 4th argument, flags, that can be
> AT_SYMLINK_NOFOLLOW. Support for changing the access mode of symbolic
> links is optional (EOPNOTSUPP allowed if not supported), but this flag
> is important even on systems where symlinks do not have access modes,
> since it's the only way to safely change the mode of a file which
> might be asynchronously replaced with a symbolic link, without a race
> condition whereby the link target is changed.

Could we also add AT_EMPTY_PATH support, so that you can fchmod an open
file in a race-free way without needing to go through procfs?

> It's possible to emulate AT_SYMLINK_NOFOLLOW in userspace, and both
> musl libc and glibc do this, by opening an O_PATH file descriptor and
> performing chmod on the corresponding magic symlink in /proc/self/fd.
> However, this requires procfs to be mounted and accessible.
> 
> Signed-off-by: Rich Felker <dalias@libc.org>
> ---
>  arch/alpha/kernel/syscalls/syscall.tbl      |  1 +
>  arch/arm/tools/syscall.tbl                  |  1 +
>  arch/arm64/include/asm/unistd.h             |  2 +-
>  arch/arm64/include/asm/unistd32.h           |  2 ++
>  arch/ia64/kernel/syscalls/syscall.tbl       |  1 +
>  arch/m68k/kernel/syscalls/syscall.tbl       |  1 +
>  arch/microblaze/kernel/syscalls/syscall.tbl |  1 +
>  arch/mips/kernel/syscalls/syscall_n32.tbl   |  1 +
>  arch/mips/kernel/syscalls/syscall_n64.tbl   |  1 +
>  arch/mips/kernel/syscalls/syscall_o32.tbl   |  1 +
>  arch/parisc/kernel/syscalls/syscall.tbl     |  1 +
>  arch/powerpc/kernel/syscalls/syscall.tbl    |  1 +
>  arch/s390/kernel/syscalls/syscall.tbl       |  1 +
>  arch/sh/kernel/syscalls/syscall.tbl         |  1 +
>  arch/sparc/kernel/syscalls/syscall.tbl      |  1 +
>  arch/x86/entry/syscalls/syscall_32.tbl      |  1 +
>  arch/x86/entry/syscalls/syscall_64.tbl      |  1 +
>  arch/xtensa/kernel/syscalls/syscall.tbl     |  1 +
>  fs/open.c                                   | 17 ++++++++++++++---
>  include/linux/syscalls.h                    |  2 ++
>  include/uapi/asm-generic/unistd.h           |  4 +++-
>  21 files changed, 38 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
> index ec8bed9e7b75..5648fa8be7a1 100644
> --- a/arch/alpha/kernel/syscalls/syscall.tbl
> +++ b/arch/alpha/kernel/syscalls/syscall.tbl
> @@ -479,3 +479,4 @@
>  547	common	openat2				sys_openat2
>  548	common	pidfd_getfd			sys_pidfd_getfd
>  549	common	faccessat2			sys_faccessat2
> +550	common	fchmodat2			sys_fchmodat2
> diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
> index 171077cbf419..b6b715bb3315 100644
> --- a/arch/arm/tools/syscall.tbl
> +++ b/arch/arm/tools/syscall.tbl
> @@ -453,3 +453,4 @@
>  437	common	openat2				sys_openat2
>  438	common	pidfd_getfd			sys_pidfd_getfd
>  439	common	faccessat2			sys_faccessat2
> +440	common	fchmodat2			sys_fchmodat2
> diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
> index 3b859596840d..b3b2019f8d16 100644
> --- a/arch/arm64/include/asm/unistd.h
> +++ b/arch/arm64/include/asm/unistd.h
> @@ -38,7 +38,7 @@
>  #define __ARM_NR_compat_set_tls		(__ARM_NR_COMPAT_BASE + 5)
>  #define __ARM_NR_COMPAT_END		(__ARM_NR_COMPAT_BASE + 0x800)
>  
> -#define __NR_compat_syscalls		440
> +#define __NR_compat_syscalls		441
>  #endif
>  
>  #define __ARCH_WANT_SYS_CLONE
> diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
> index 734860ac7cf9..cd0845f3c19f 100644
> --- a/arch/arm64/include/asm/unistd32.h
> +++ b/arch/arm64/include/asm/unistd32.h
> @@ -887,6 +887,8 @@ __SYSCALL(__NR_openat2, sys_openat2)
>  __SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd)
>  #define __NR_faccessat2 439
>  __SYSCALL(__NR_faccessat2, sys_faccessat2)
> +#define __NR_fchmodat2 440
> +__SYSCALL(__NR_fchmodat2, sys_fchmodat2)
>  
>  /*
>   * Please add new compat syscalls above this comment and update
> diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
> index f52a41f4c340..7c3f8564d0f3 100644
> --- a/arch/ia64/kernel/syscalls/syscall.tbl
> +++ b/arch/ia64/kernel/syscalls/syscall.tbl
> @@ -360,3 +360,4 @@
>  437	common	openat2				sys_openat2
>  438	common	pidfd_getfd			sys_pidfd_getfd
>  439	common	faccessat2			sys_faccessat2
> +440	common	fchmodat2			sys_fchmodat2
> diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
> index 81fc799d8392..063d875377bf 100644
> --- a/arch/m68k/kernel/syscalls/syscall.tbl
> +++ b/arch/m68k/kernel/syscalls/syscall.tbl
> @@ -439,3 +439,4 @@
>  437	common	openat2				sys_openat2
>  438	common	pidfd_getfd			sys_pidfd_getfd
>  439	common	faccessat2			sys_faccessat2
> +440	common	fchmodat2			sys_fchmodat2
> diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
> index b4e263916f41..6aea8a435fd0 100644
> --- a/arch/microblaze/kernel/syscalls/syscall.tbl
> +++ b/arch/microblaze/kernel/syscalls/syscall.tbl
> @@ -445,3 +445,4 @@
>  437	common	openat2				sys_openat2
>  438	common	pidfd_getfd			sys_pidfd_getfd
>  439	common	faccessat2			sys_faccessat2
> +440	common	fchmodat2			sys_fchmodat2
> diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
> index f9df9edb67a4..a9205843251d 100644
> --- a/arch/mips/kernel/syscalls/syscall_n32.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
> @@ -378,3 +378,4 @@
>  437	n32	openat2				sys_openat2
>  438	n32	pidfd_getfd			sys_pidfd_getfd
>  439	n32	faccessat2			sys_faccessat2
> +440	n32	fchmodat2			sys_fchmodat2
> diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
> index 557f9954a2b9..31da28e2d6f3 100644
> --- a/arch/mips/kernel/syscalls/syscall_n64.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
> @@ -354,3 +354,4 @@
>  437	n64	openat2				sys_openat2
>  438	n64	pidfd_getfd			sys_pidfd_getfd
>  439	n64	faccessat2			sys_faccessat2
> +440	n64	fchmodat2			sys_fchmodat2
> diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
> index 195b43cf27c8..af0e38302ed8 100644
> --- a/arch/mips/kernel/syscalls/syscall_o32.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
> @@ -427,3 +427,4 @@
>  437	o32	openat2				sys_openat2
>  438	o32	pidfd_getfd			sys_pidfd_getfd
>  439	o32	faccessat2			sys_faccessat2
> +440	o32	fchmodat2			sys_fchmodat2
> diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
> index def64d221cd4..379cdb44ca0b 100644
> --- a/arch/parisc/kernel/syscalls/syscall.tbl
> +++ b/arch/parisc/kernel/syscalls/syscall.tbl
> @@ -437,3 +437,4 @@
>  437	common	openat2				sys_openat2
>  438	common	pidfd_getfd			sys_pidfd_getfd
>  439	common	faccessat2			sys_faccessat2
> +440	common	fchmodat2			sys_fchmodat2
> diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
> index c2d737ff2e7b..ada11db506e6 100644
> --- a/arch/powerpc/kernel/syscalls/syscall.tbl
> +++ b/arch/powerpc/kernel/syscalls/syscall.tbl
> @@ -529,3 +529,4 @@
>  437	common	openat2				sys_openat2
>  438	common	pidfd_getfd			sys_pidfd_getfd
>  439	common	faccessat2			sys_faccessat2
> +440	common	fchmodat2			sys_fchmodat2
> diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
> index 10456bc936fb..a4dae0abb353 100644
> --- a/arch/s390/kernel/syscalls/syscall.tbl
> +++ b/arch/s390/kernel/syscalls/syscall.tbl
> @@ -442,3 +442,4 @@
>  437  common	openat2			sys_openat2			sys_openat2
>  438  common	pidfd_getfd		sys_pidfd_getfd			sys_pidfd_getfd
>  439  common	faccessat2		sys_faccessat2			sys_faccessat2
> +440  common	fchmodat2		sys_fchmodat2			sys_fchmodat2
> diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
> index ae0a00beea5f..b59b4408b85f 100644
> --- a/arch/sh/kernel/syscalls/syscall.tbl
> +++ b/arch/sh/kernel/syscalls/syscall.tbl
> @@ -442,3 +442,4 @@
>  437	common	openat2				sys_openat2
>  438	common	pidfd_getfd			sys_pidfd_getfd
>  439	common	faccessat2			sys_faccessat2
> +440	common	fchmodat2			sys_fchmodat2
> diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
> index 4af114e84f20..e817416f81df 100644
> --- a/arch/sparc/kernel/syscalls/syscall.tbl
> +++ b/arch/sparc/kernel/syscalls/syscall.tbl
> @@ -485,3 +485,4 @@
>  437	common	openat2			sys_openat2
>  438	common	pidfd_getfd			sys_pidfd_getfd
>  439	common	faccessat2			sys_faccessat2
> +440	common	fchmodat2			sys_fchmodat2
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 9d1102873666..208b06650cef 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -444,3 +444,4 @@
>  437	i386	openat2			sys_openat2
>  438	i386	pidfd_getfd		sys_pidfd_getfd
>  439	i386	faccessat2		sys_faccessat2
> +440	i386	fchmodat2		sys_fchmodat2
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index f30d6ae9a688..d9a591db72fb 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -361,6 +361,7 @@
>  437	common	openat2			sys_openat2
>  438	common	pidfd_getfd		sys_pidfd_getfd
>  439	common	faccessat2		sys_faccessat2
> +440	common	fchmodat2		sys_fchmodat2
>  
>  #
>  # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
> index 6276e3c2d3fc..ff756cb2f5d7 100644
> --- a/arch/xtensa/kernel/syscalls/syscall.tbl
> +++ b/arch/xtensa/kernel/syscalls/syscall.tbl
> @@ -410,3 +410,4 @@
>  437	common	openat2				sys_openat2
>  438	common	pidfd_getfd			sys_pidfd_getfd
>  439	common	faccessat2			sys_faccessat2
> +440	common	fchmodat2			sys_fchmodat2
> diff --git a/fs/open.c b/fs/open.c
> index cdb7964aaa6e..f492c782c0ed 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -616,11 +616,16 @@ 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 flags)
>  {
>  	struct path path;
>  	int error;
>  	unsigned int lookup_flags = LOOKUP_FOLLOW;
> +
> +	if (flags & ~AT_SYMLINK_NOFOLLOW)
> +		return -EINVAL;
> +	if (flags & AT_SYMLINK_NOFOLLOW)
> +		lookup_flags &= ~LOOKUP_FOLLOW;
>  retry:
>  	error = user_path_at(dfd, filename, lookup_flags, &path);
>  	if (!error) {
> @@ -634,15 +639,21 @@ 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)
> +{
> +	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);
>  }
>  
>  int chown_common(const struct path *path, uid_t user, gid_t group)
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 75ac7f8ae93c..ced00c56eba7 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -435,6 +435,8 @@ 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);
> diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> index 995b36c2ea7d..ebf5cdb3f444 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -859,9 +859,11 @@ __SYSCALL(__NR_openat2, sys_openat2)
>  __SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd)
>  #define __NR_faccessat2 439
>  __SYSCALL(__NR_faccessat2, sys_faccessat2)
> +#define __NR_fchmodat2 440
> +__SYSCALL(__NR_fchmodat2, sys_fchmodat2)
>  
>  #undef __NR_syscalls
> -#define __NR_syscalls 440
> +#define __NR_syscalls 441
>  
>  /*
>   * 32 bit systems traditionally used different
> -- 
> 2.21.0
>
Greg Kroah-Hartman Sept. 16, 2020, 6:19 a.m. UTC | #2
On Tue, Sep 15, 2020 at 08:23:38PM -0400, Rich Felker wrote:
> POSIX defines fchmodat as having a 4th argument, flags, that can be
> AT_SYMLINK_NOFOLLOW. Support for changing the access mode of symbolic
> links is optional (EOPNOTSUPP allowed if not supported), but this flag
> is important even on systems where symlinks do not have access modes,
> since it's the only way to safely change the mode of a file which
> might be asynchronously replaced with a symbolic link, without a race
> condition whereby the link target is changed.
> 
> It's possible to emulate AT_SYMLINK_NOFOLLOW in userspace, and both
> musl libc and glibc do this, by opening an O_PATH file descriptor and
> performing chmod on the corresponding magic symlink in /proc/self/fd.
> However, this requires procfs to be mounted and accessible.
> 
> Signed-off-by: Rich Felker <dalias@libc.org>

No kselftest for this new system call, or man page?  How do we know this
actually works and what the expected outcome should be?

thanks,

greg k-h
diff mbox series

Patch

diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index ec8bed9e7b75..5648fa8be7a1 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl
@@ -479,3 +479,4 @@ 
 547	common	openat2				sys_openat2
 548	common	pidfd_getfd			sys_pidfd_getfd
 549	common	faccessat2			sys_faccessat2
+550	common	fchmodat2			sys_fchmodat2
diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index 171077cbf419..b6b715bb3315 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -453,3 +453,4 @@ 
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	faccessat2			sys_faccessat2
+440	common	fchmodat2			sys_fchmodat2
diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
index 3b859596840d..b3b2019f8d16 100644
--- a/arch/arm64/include/asm/unistd.h
+++ b/arch/arm64/include/asm/unistd.h
@@ -38,7 +38,7 @@ 
 #define __ARM_NR_compat_set_tls		(__ARM_NR_COMPAT_BASE + 5)
 #define __ARM_NR_COMPAT_END		(__ARM_NR_COMPAT_BASE + 0x800)
 
-#define __NR_compat_syscalls		440
+#define __NR_compat_syscalls		441
 #endif
 
 #define __ARCH_WANT_SYS_CLONE
diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
index 734860ac7cf9..cd0845f3c19f 100644
--- a/arch/arm64/include/asm/unistd32.h
+++ b/arch/arm64/include/asm/unistd32.h
@@ -887,6 +887,8 @@  __SYSCALL(__NR_openat2, sys_openat2)
 __SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd)
 #define __NR_faccessat2 439
 __SYSCALL(__NR_faccessat2, sys_faccessat2)
+#define __NR_fchmodat2 440
+__SYSCALL(__NR_fchmodat2, sys_fchmodat2)
 
 /*
  * Please add new compat syscalls above this comment and update
diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
index f52a41f4c340..7c3f8564d0f3 100644
--- a/arch/ia64/kernel/syscalls/syscall.tbl
+++ b/arch/ia64/kernel/syscalls/syscall.tbl
@@ -360,3 +360,4 @@ 
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	faccessat2			sys_faccessat2
+440	common	fchmodat2			sys_fchmodat2
diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
index 81fc799d8392..063d875377bf 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl
@@ -439,3 +439,4 @@ 
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	faccessat2			sys_faccessat2
+440	common	fchmodat2			sys_fchmodat2
diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index b4e263916f41..6aea8a435fd0 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl
@@ -445,3 +445,4 @@ 
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	faccessat2			sys_faccessat2
+440	common	fchmodat2			sys_fchmodat2
diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
index f9df9edb67a4..a9205843251d 100644
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
@@ -378,3 +378,4 @@ 
 437	n32	openat2				sys_openat2
 438	n32	pidfd_getfd			sys_pidfd_getfd
 439	n32	faccessat2			sys_faccessat2
+440	n32	fchmodat2			sys_fchmodat2
diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
index 557f9954a2b9..31da28e2d6f3 100644
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
@@ -354,3 +354,4 @@ 
 437	n64	openat2				sys_openat2
 438	n64	pidfd_getfd			sys_pidfd_getfd
 439	n64	faccessat2			sys_faccessat2
+440	n64	fchmodat2			sys_fchmodat2
diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
index 195b43cf27c8..af0e38302ed8 100644
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
@@ -427,3 +427,4 @@ 
 437	o32	openat2				sys_openat2
 438	o32	pidfd_getfd			sys_pidfd_getfd
 439	o32	faccessat2			sys_faccessat2
+440	o32	fchmodat2			sys_fchmodat2
diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index def64d221cd4..379cdb44ca0b 100644
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl
@@ -437,3 +437,4 @@ 
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	faccessat2			sys_faccessat2
+440	common	fchmodat2			sys_fchmodat2
diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index c2d737ff2e7b..ada11db506e6 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -529,3 +529,4 @@ 
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	faccessat2			sys_faccessat2
+440	common	fchmodat2			sys_fchmodat2
diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index 10456bc936fb..a4dae0abb353 100644
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl
@@ -442,3 +442,4 @@ 
 437  common	openat2			sys_openat2			sys_openat2
 438  common	pidfd_getfd		sys_pidfd_getfd			sys_pidfd_getfd
 439  common	faccessat2		sys_faccessat2			sys_faccessat2
+440  common	fchmodat2		sys_fchmodat2			sys_fchmodat2
diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
index ae0a00beea5f..b59b4408b85f 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl
@@ -442,3 +442,4 @@ 
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	faccessat2			sys_faccessat2
+440	common	fchmodat2			sys_fchmodat2
diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index 4af114e84f20..e817416f81df 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl
@@ -485,3 +485,4 @@ 
 437	common	openat2			sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	faccessat2			sys_faccessat2
+440	common	fchmodat2			sys_fchmodat2
diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 9d1102873666..208b06650cef 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -444,3 +444,4 @@ 
 437	i386	openat2			sys_openat2
 438	i386	pidfd_getfd		sys_pidfd_getfd
 439	i386	faccessat2		sys_faccessat2
+440	i386	fchmodat2		sys_fchmodat2
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index f30d6ae9a688..d9a591db72fb 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -361,6 +361,7 @@ 
 437	common	openat2			sys_openat2
 438	common	pidfd_getfd		sys_pidfd_getfd
 439	common	faccessat2		sys_faccessat2
+440	common	fchmodat2		sys_fchmodat2
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact
diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
index 6276e3c2d3fc..ff756cb2f5d7 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl
@@ -410,3 +410,4 @@ 
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	faccessat2			sys_faccessat2
+440	common	fchmodat2			sys_fchmodat2
diff --git a/fs/open.c b/fs/open.c
index cdb7964aaa6e..f492c782c0ed 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -616,11 +616,16 @@  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 flags)
 {
 	struct path path;
 	int error;
 	unsigned int lookup_flags = LOOKUP_FOLLOW;
+
+	if (flags & ~AT_SYMLINK_NOFOLLOW)
+		return -EINVAL;
+	if (flags & AT_SYMLINK_NOFOLLOW)
+		lookup_flags &= ~LOOKUP_FOLLOW;
 retry:
 	error = user_path_at(dfd, filename, lookup_flags, &path);
 	if (!error) {
@@ -634,15 +639,21 @@  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)
+{
+	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);
 }
 
 int chown_common(const struct path *path, uid_t user, gid_t group)
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 75ac7f8ae93c..ced00c56eba7 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -435,6 +435,8 @@  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);
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 995b36c2ea7d..ebf5cdb3f444 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -859,9 +859,11 @@  __SYSCALL(__NR_openat2, sys_openat2)
 __SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd)
 #define __NR_faccessat2 439
 __SYSCALL(__NR_faccessat2, sys_faccessat2)
+#define __NR_fchmodat2 440
+__SYSCALL(__NR_fchmodat2, sys_fchmodat2)
 
 #undef __NR_syscalls
-#define __NR_syscalls 440
+#define __NR_syscalls 441
 
 /*
  * 32 bit systems traditionally used different