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-9c81-8e21ea1f9...@xen0n.name/ Cc: Christian Brauner <brau...@kernel.org> Cc: Alexander Viro <v...@zeniv.linux.org.uk> Cc: Jan Kara <j...@suse.cz> Cc: Steven Rostedt <rost...@goodmis.org> Cc: Masami Hiramatsu <mhira...@kernel.org> Cc: Alejandro Colomar <a...@kernel.org> Cc: Arnd Bergmann <a...@arndb.de> Cc: Huacai Chen <chenhua...@loongson.cn> Cc: Xuerui Wang <ker...@xen0n.name> Cc: Jiaxun Yang <jiaxun.y...@flygoat.com> Cc: Icenowy Zheng <u...@icenowy.me> Cc: linux-fsde...@vger.kernel.org Cc: linux-trace-ker...@vger.kernel.org Cc: linux-a...@vger.kernel.org Cc: loonga...@lists.linux.dev Cc: linux-kernel@vger.kernel.org Signed-off-by: Xi Ruoyao <xry...@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
