[PATCH 0/2] vfs: POSIX-compliant version of fchmodat with flag argument

[PATCH 0/2] vfs: POSIX-compliant version of fchmodat with flag argument

[Andrew Ayer]

Hi,

The below patches implement a 4 argument version of fchmodat (fchmodat4)
that supports a flag argument, as specified by POSIX.  Currently, the
glibc wrapper has a flag argument, but fails with EOPNOTSUPP if you
specify AT_SYMLINK_NOFOLLOW. POSIX says that fchmodat should only fail
with EOPNOTSUPP if you specify AT_SYMLINK_NOFOLLOW and the path actually
refers to a symbolic link (and the system doesn't support changing the
mode of symbolic links).  Unfortunately glibc can't do better because
the current syscall doesn't have a flag argument.

In addition to supporting AT_SYMLINK_NOFOLLOW, this also supports the
AT_EMPTY_PATH flag, just like fchownat.

Besides the POSIX-compliance, this patch will make it possible to
ensure, in a race-free way, that you do not follow symlinks when
chmodding. Previously, you could open a file with O_NOFOLLOW and fchmod
it, but this only worked if you had read or write permissions on the
file.  Now you can open the file with O_PATH|O_NOFOLLOW and do fchmodat
with AT_EMPTY_PATH.

Any feedback is welcome.

Cheers, 
Andrew

[PATCH 1/2] vfs: Add POSIX-compliant version of fchmodat syscall

[Andrew Ayer]

This adds a 4 argument version of fchmodat (fchmodat4) that
supports a flag argument, as specified by POSIX.  It supports
the same two flags as fchownat: AT_SYMLINK_NOFOLLOW and AT_EMPTY_PATH.

Signed-off-by: Andrew Ayer <agwa@andrewayer.name>
---
 fs/open.c                |   21 +++++++++++++++++++--
 include/linux/syscalls.h |    2 ++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 22c41b5..c3dfdaa 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -452,6 +452,9 @@ static int chmod_common(struct path *path, umode_t mode)
 	struct iattr newattrs;
 	int error;
 
+	if (S_ISLNK(inode->i_mode))
+		return -EOPNOTSUPP;
+
 	error = mnt_want_write(path->mnt);
 	if (error)
 		return error;
@@ -484,10 +487,24 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd, mode_t, mode)
 
 SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename, mode_t, mode)
 {
+	return sys_fchmodat4(dfd, filename, mode, 0);
+}
+
+SYSCALL_DEFINE4(fchmodat4, int, dfd, const char __user *, filename,
+		mode_t, mode, int, flag)
+{
 	struct path path;
 	int error;
+	int lookup_flags;
+
+	if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
+		return -EINVAL;
 
-	error = user_path_at(dfd, filename, LOOKUP_FOLLOW, &path);
+	lookup_flags = (flag & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW;
+	if (flag & AT_EMPTY_PATH)
+		lookup_flags |= LOOKUP_EMPTY;
+
+	error = user_path_at(dfd, filename, lookup_flags, &path);
 	if (!error) {
 		error = chmod_common(&path, mode);
 		path_put(&path);
@@ -497,7 +514,7 @@ SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename, mode_t, mode)
 
 SYSCALL_DEFINE2(chmod, const char __user *, filename, mode_t, mode)
 {
-	return sys_fchmodat(AT_FDCWD, filename, mode);
+	return sys_fchmodat4(AT_FDCWD, filename, mode, 0);
 }
 
 static int chown_common(struct path *path, uid_t user, gid_t group)
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 86a24b1..c84795b 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -770,6 +770,8 @@ asmlinkage long sys_futimesat(int dfd, const char __user *filename,
 asmlinkage long sys_faccessat(int dfd, const char __user *filename, int mode);
 asmlinkage long sys_fchmodat(int dfd, const char __user * filename,
 			     mode_t mode);
+asmlinkage long sys_fchmodat4(int dfd, const char __user * filename,
+			     mode_t mode, int flag);
 asmlinkage long sys_fchownat(int dfd, const char __user *filename, uid_t user,
 			     gid_t group, int flag);
 asmlinkage long sys_openat(int dfd, const char __user *filename, int flags,
-- 
1.7.1

[PATCH 2/2] x86: Hook up new fchmodat4 syscall

[Andrew Ayer]

Signed-off-by: Andrew Ayer <agwa@andrewayer.name>
---
 arch/x86/ia32/ia32entry.S          |    1 +
 arch/x86/include/asm/unistd_32.h   |    3 ++-
 arch/x86/include/asm/unistd_64.h   |    2 ++
 arch/x86/kernel/syscall_table_32.S |    1 +
 4 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index a6253ec..32248c5 100644
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -852,4 +852,5 @@ ia32_sys_call_table:
 	.quad sys_setns
 	.quad compat_sys_process_vm_readv
 	.quad compat_sys_process_vm_writev
+	.quad sys_fchmodat4
 ia32_syscall_end:
diff --git a/arch/x86/include/asm/unistd_32.h b/arch/x86/include/asm/unistd_32.h
index 599c77d..112b299 100644
--- a/arch/x86/include/asm/unistd_32.h
+++ b/arch/x86/include/asm/unistd_32.h
@@ -354,10 +354,11 @@
 #define __NR_setns		346
 #define __NR_process_vm_readv	347
 #define __NR_process_vm_writev	348
+#define __NR_fchmodat4		349
 
 #ifdef __KERNEL__
 
-#define NR_syscalls 349
+#define NR_syscalls 350
 
 #define __ARCH_WANT_IPC_PARSE_VERSION
 #define __ARCH_WANT_OLD_READDIR
diff --git a/arch/x86/include/asm/unistd_64.h b/arch/x86/include/asm/unistd_64.h
index 0431f19..21564f9 100644
--- a/arch/x86/include/asm/unistd_64.h
+++ b/arch/x86/include/asm/unistd_64.h
@@ -686,6 +686,8 @@ __SYSCALL(__NR_getcpu, sys_getcpu)
 __SYSCALL(__NR_process_vm_readv, sys_process_vm_readv)
 #define __NR_process_vm_writev			311
 __SYSCALL(__NR_process_vm_writev, sys_process_vm_writev)
+#define __NR_fchmodat4				312
+__SYSCALL(__NR_fchmodat4, sys_fchmodat4)
 
 #ifndef __NO_STUBS
 #define __ARCH_WANT_OLD_READDIR
diff --git a/arch/x86/kernel/syscall_table_32.S b/arch/x86/kernel/syscall_table_32.S
index 9a0e312..bff82a6 100644
--- a/arch/x86/kernel/syscall_table_32.S
+++ b/arch/x86/kernel/syscall_table_32.S
@@ -348,3 +348,4 @@ ENTRY(sys_call_table)
 	.long sys_setns
 	.long sys_process_vm_readv
 	.long sys_process_vm_writev
+	.long sys_fchmodat4
-- 
1.7.1

[PATCH 2/2] x86: Hook up new fchmodat4 syscall

[Andrew Ayer]

Signed-off-by: Andrew Ayer <agwa@andrewayer.name>
---
 arch/x86/ia32/ia32entry.S          |    1 +
 arch/x86/include/asm/unistd_32.h   |    3 ++-
 arch/x86/include/asm/unistd_64.h   |    2 ++
 arch/x86/kernel/syscall_table_32.S |    1 +
 4 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index 3e27456..f026e9f 100644
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -847,4 +847,5 @@ ia32_sys_call_table:
 	.quad sys_setns
 	.quad compat_sys_process_vm_readv
 	.quad compat_sys_process_vm_writev
+	.quad sys_fchmodat4
 ia32_syscall_end:
diff --git a/arch/x86/include/asm/unistd_32.h b/arch/x86/include/asm/unistd_32.h
index 599c77d..112b299 100644
--- a/arch/x86/include/asm/unistd_32.h
+++ b/arch/x86/include/asm/unistd_32.h
@@ -354,10 +354,11 @@
 #define __NR_setns		346
 #define __NR_process_vm_readv	347
 #define __NR_process_vm_writev	348
+#define __NR_fchmodat4		349
 
 #ifdef __KERNEL__
 
-#define NR_syscalls 349
+#define NR_syscalls 350
 
 #define __ARCH_WANT_IPC_PARSE_VERSION
 #define __ARCH_WANT_OLD_READDIR
diff --git a/arch/x86/include/asm/unistd_64.h b/arch/x86/include/asm/unistd_64.h
index 0431f19..21564f9 100644
--- a/arch/x86/include/asm/unistd_64.h
+++ b/arch/x86/include/asm/unistd_64.h
@@ -686,6 +686,8 @@ __SYSCALL(__NR_getcpu, sys_getcpu)
 __SYSCALL(__NR_process_vm_readv, sys_process_vm_readv)
 #define __NR_process_vm_writev			311
 __SYSCALL(__NR_process_vm_writev, sys_process_vm_writev)
+#define __NR_fchmodat4				312
+__SYSCALL(__NR_fchmodat4, sys_fchmodat4)
 
 #ifndef __NO_STUBS
 #define __ARCH_WANT_OLD_READDIR
diff --git a/arch/x86/kernel/syscall_table_32.S b/arch/x86/kernel/syscall_table_32.S
index 9a0e312..bff82a6 100644
--- a/arch/x86/kernel/syscall_table_32.S
+++ b/arch/x86/kernel/syscall_table_32.S
@@ -348,3 +348,4 @@ ENTRY(sys_call_table)
 	.long sys_setns
 	.long sys_process_vm_readv
 	.long sys_process_vm_writev
+	.long sys_fchmodat4
-- 
1.7.1