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Message-ID: <20241008042751.GW4017910@ZenIV>
Date: Tue, 8 Oct 2024 05:27:51 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Xi Ruoyao <xry111@...111.site>, Christian Brauner <brauner@...nel.org>,
Miao Wang <shankerwangmiao@...il.com>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
stable@...r.kernel.org
Subject: Re: [PATCH 2/2] vfs: Make sure {statx,fstatat}(..., AT_EMPTY_PATH |
..., NULL, ...) behave as (..., AT_EMPTY_PATH | ..., "", ...)
On Tue, Oct 08, 2024 at 05:16:21AM +0100, Al Viro wrote:
> Folks, please don't go there. Really. IMO vfs_empty_path() is a wrong API
> in the first place. Too low-level and racy as well.
>
> See the approach in #work.xattr; I'm going to lift that into fs/namei.c
> (well, the slow path - everything after "if path is NULL, we are done") and
> yes, fs/stat.c users get handled better that way.
FWIW, the intermediate (just after that commit) state of those functions is
int vfs_fstatat(int dfd, const char __user *filename,
struct kstat *stat, int flags)
{
int ret;
int statx_flags = flags | AT_NO_AUTOMOUNT;
struct filename *name = getname_maybe_null(filename, flags);
if (!name)
return vfs_fstat(dfd, stat);
ret = vfs_statx(dfd, name, statx_flags, stat, STATX_BASIC_STATS);
putname(name);
return ret;
}
and
SYSCALL_DEFINE5(statx,
int, dfd, const char __user *, filename, unsigned, flags,
unsigned int, mask,
struct statx __user *, buffer)
{
int ret;
unsigned lflags;
struct filename *name = getname_maybe_null(filename, flags);
/*
* Short-circuit handling of NULL and "" paths.
*
* For a NULL path we require and accept only the AT_EMPTY_PATH flag
* (possibly |'d with AT_STATX flags).
*
* However, glibc on 32-bit architectures implements fstatat as statx
* with the "" pathname and AT_NO_AUTOMOUNT | AT_EMPTY_PATH flags.
* Supporting this results in the uglification below.
*/
lflags = flags & ~(AT_NO_AUTOMOUNT | AT_STATX_SYNC_TYPE);
if (!name)
return do_statx_fd(dfd, flags & ~AT_NO_AUTOMOUNT, mask, buffer);
ret = do_statx(dfd, name, flags, mask, buffer);
putname(name);
return ret;
}
static inline struct filename *getname_maybe_null(const char __user *name, int flags)
{
if (!(flags & AT_EMPTY_PATH))
return getname(name);
if (!name)
return NULL;
return __getname_maybe_null(name);
}
struct filename *__getname_maybe_null(const char __user *pathname)
{
struct filename *name;
char c;
/* try to save on allocations; loss on um, though */
if (get_user(c, pathname))
return ERR_PTR(-EFAULT);
if (!c)
return NULL;
name = getname_flags(pathname, LOOKUP_EMPTY);
if (!IS_ERR(name) && !(name->name[0])) {
putname(name);
name = NULL;
}
return name;
}
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