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Date:   Fri, 18 Oct 2019 09:55:12 +0200
From:   Miklos Szeredi <miklos@szeredi.hu>
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH] f*xattr: allow O_PATH descriptors
Message-ID: <20191018075512.GA2075@miu.piliscsaba.redhat.com>
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From: Miklos Szeredi <mszeredi@redhat.com>

This allows xattr ops on symlink/special files referenced by an O_PATH
descriptor without having to play games with /proc/self/fd/NN (which
doesn't work for symlinks anyway).

This capability is the same as would be given by introducing ...at()
variants with an AT_EMPTY_PATH argument.  Looking at getattr/setattr type
syscalls, this is allowed for fstatat() and fchownat(), but not for
fchmodat() and utimensat().  What's the logic?

While this carries a minute risk of someone relying on the property of
xattr syscalls rejecting O_PATH descriptors, it saves the trouble of
introducing another set of syscalls.

Only file->f_path and file->f_inode are accessed in these functions.

Current versions return EBADF, hence easy to detect the presense of this
feature and fall back in case it's missing.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 fs/xattr.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -495,7 +495,7 @@ SYSCALL_DEFINE5(lsetxattr, const char __
 SYSCALL_DEFINE5(fsetxattr, int, fd, const char __user *, name,
 		const void __user *,value, size_t, size, int, flags)
 {
-	struct fd f = fdget(fd);
+	struct fd f = fdget_raw(fd);
 	int error = -EBADF;
 
 	if (!f.file)
@@ -587,7 +587,7 @@ SYSCALL_DEFINE4(lgetxattr, const char __
 SYSCALL_DEFINE4(fgetxattr, int, fd, const char __user *, name,
 		void __user *, value, size_t, size)
 {
-	struct fd f = fdget(fd);
+	struct fd f = fdget_raw(fd);
 	ssize_t error = -EBADF;
 
 	if (!f.file)
@@ -662,7 +662,7 @@ SYSCALL_DEFINE3(llistxattr, const char _
 
 SYSCALL_DEFINE3(flistxattr, int, fd, char __user *, list, size_t, size)
 {
-	struct fd f = fdget(fd);
+	struct fd f = fdget_raw(fd);
 	ssize_t error = -EBADF;
 
 	if (!f.file)
@@ -727,7 +727,7 @@ SYSCALL_DEFINE2(lremovexattr, const char
 
 SYSCALL_DEFINE2(fremovexattr, int, fd, const char __user *, name)
 {
-	struct fd f = fdget(fd);
+	struct fd f = fdget_raw(fd);
 	int error = -EBADF;
 
 	if (!f.file)