[PATCH] vfs: Add AT_EMPTY_PATH_NOCHECK as unchecked AT_EMPTY_PATH

[Xi Ruoyao <xry111@xry111.site>]

It's cheap to check if the path is empty in the userspace, but expensive
to check if a userspace string is empty from the kernel.  So using statx
and AT_EMPTY_PATH to implement fstat is slower than a "native" fstat
call.  But for arch/loongarch fstat does not exist so we have to use
statx, and on all 32-bit architectures we must use statx after 2037.
And seccomp also cannot audit AT_EMPTY_PATH properly because it cannot
check if path is empty.

To resolve these issues, add a relaxed version of AT_EMPTY_PATH: it does
not check if the path is empty, but just assumes the path is empty and
then behaves like AT_EMPTY_PATH.

Link: https://sourceware.org/pipermail/libc-alpha/2023-September/151364.html
Link: https://lore.kernel.org/loongarch/599df4a3-47a4-49be-9c81-8e21ea1f988a@xen0n.name/
Cc: Christian Brauner <brauner@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jan Kara <jack@suse.cz>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Alejandro Colomar <alx@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Huacai Chen <chenhuacai@loongson.cn>
Cc: Xuerui Wang <kernel@xen0n.name>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Icenowy Zheng <uwu@icenowy.me>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-trace-kernel@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Cc: loongarch@lists.linux.dev
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
---
 fs/namei.c                 | 8 +++++++-
 fs/stat.c                  | 4 +++-
 include/linux/namei.h      | 4 ++++
 include/trace/misc/fs.h    | 1 +
 include/uapi/linux/fcntl.h | 3 +++
 5 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 37fb0a8aa09a..0c44a7ea5961 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -147,7 +147,13 @@ getname_flags(const char __user *filename, int flags, int *empty)
 	kname = (char *)result->iname;
 	result->name = kname;
 
-	len = strncpy_from_user(kname, filename, EMBEDDED_NAME_MAX);
+	if (!(flags & LOOKUP_EMPTY_NOCHECK))
+		len = strncpy_from_user(kname, filename, EMBEDDED_NAME_MAX);
+	else {
+		len = 0;
+		kname[0] = '\0';
+	}
+
 	if (unlikely(len < 0)) {
 		__putname(result);
 		return ERR_PTR(len);
diff --git a/fs/stat.c b/fs/stat.c
index 70bd3e888cfa..53944d3287cd 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -210,6 +210,8 @@ int getname_statx_lookup_flags(int flags)
 		lookup_flags |= LOOKUP_AUTOMOUNT;
 	if (flags & AT_EMPTY_PATH)
 		lookup_flags |= LOOKUP_EMPTY;
+	if (flags & AT_EMPTY_PATH_NOCHECK)
+		lookup_flags |= LOOKUP_EMPTY | LOOKUP_EMPTY_NOCHECK;
 
 	return lookup_flags;
 }
@@ -237,7 +239,7 @@ static int vfs_statx(int dfd, struct filename *filename, int flags,
 	int error;
 
 	if (flags & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT | AT_EMPTY_PATH |
-		      AT_STATX_SYNC_TYPE))
+		      AT_STATX_SYNC_TYPE | AT_EMPTY_PATH_NOCHECK))
 		return -EINVAL;
 
 retry:
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 967aa9ea9f96..def6a8a1b531 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -45,9 +45,13 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT};
 #define LOOKUP_IN_ROOT		0x100000 /* Treat dirfd as fs root. */
 #define LOOKUP_CACHED		0x200000 /* Only do cached lookup */
 #define LOOKUP_LINKAT_EMPTY	0x400000 /* Linkat request with empty path. */
+
 /* LOOKUP_* flags which do scope-related checks based on the dirfd. */
 #define LOOKUP_IS_SCOPED (LOOKUP_BENEATH | LOOKUP_IN_ROOT)
 
+/* If this is set, LOOKUP_EMPTY must be set as well. */
+#define LOOKUP_EMPTY_NOCHECK	0x800000 /* Consider path empty. */
+
 extern int path_pts(struct path *path);
 
 extern int user_path_at_empty(int, const char __user *, unsigned, struct path *, int *empty);
diff --git a/include/trace/misc/fs.h b/include/trace/misc/fs.h
index 738b97f22f36..24aec7ed6b0b 100644
--- a/include/trace/misc/fs.h
+++ b/include/trace/misc/fs.h
@@ -119,4 +119,5 @@
 		{ LOOKUP_NO_XDEV,	"NO_XDEV" }, \
 		{ LOOKUP_BENEATH,	"BENEATH" }, \
 		{ LOOKUP_IN_ROOT,	"IN_ROOT" }, \
+		{ LOOKUP_EMPTY_NOCHECK,	"EMPTY_NOCHECK" }, \
 		{ LOOKUP_CACHED,	"CACHED" })
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index c0bcc185fa48..aa2f68d80820 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -113,6 +113,9 @@
 #define AT_STATX_DONT_SYNC	0x4000	/* - Don't sync attributes with the server */
 
 #define AT_RECURSIVE		0x8000	/* Apply to the entire subtree */
+#define AT_EMPTY_PATH_NOCHECK	0x10000	/* Like AT_EMPTY_PATH, but the path
+                                           is not checked and it's just
+                                           assumed to be empty */
 
 /* Flags for name_to_handle_at(2). We reuse AT_ flag space to save bits... */
 #define AT_HANDLE_FID		AT_REMOVEDIR	/* file handle is needed to
-- 
2.45.2