Re: [PATCH v2 2/2] vfs: add fchmodat2 syscall

From: Aleksa Sarai
Date: Wed Sep 16 2020 - 02:02:14 EST

On 2020-09-15, Rich Felker <dalias@xxxxxxxx> 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@xxxxxxxx>
> ---
> 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
>

--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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