linux-kernel - [PATCH] fat: Use pointer to d_name[0] in put

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Message-ID: <YgmB01p+p45Cihhg@p100>
Date:   Sun, 13 Feb 2022 23:10:27 +0100
From:   Helge Deller <deller@....de>
To:     OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-parisc@...r.kernel.org
Subject: [PATCH] fat: Use pointer to d_name[0] in put_user() for compat case

The put_user(val,ptr) macro wants a pointer in the second parameter, but in
fat_ioctl_filldir() the d_name field references a whole "array of chars".
Usually the compiler automatically converts it and uses a pointer to that
array, but it's more clean to explicitly give the real pointer to where someting
is put, which is in this case the first character of the d_name[] array.

I noticed that issue while trying to optimize the parisc put_user() macro
and used an intermediate variable to store the pointer. In that case I
got this error:

In file included from include/linux/uaccess.h:11,
                 from include/linux/compat.h:17,
                 from fs/fat/dir.c:18:
fs/fat/dir.c: In function ‘fat_ioctl_filldir’:
fs/fat/dir.c:725:33: error: invalid initializer
  725 |                 if (put_user(0, d2->d_name)                     ||         \
      |                                 ^~
include/asm/uaccess.h:152:33: note: in definition of macro ‘__put_user’
  152 |         __typeof__(ptr) __ptr = ptr;                            \
      |                                 ^~~
fs/fat/dir.c:759:1: note: in expansion of macro ‘FAT_IOCTL_FILLDIR_FUNC’
  759 | FAT_IOCTL_FILLDIR_FUNC(fat_ioctl_filldir, __fat_dirent)

The patch below cleans it up.

Signed-off-by: Helge Deller <deller@....de>

diff --git a/fs/fat/dir.c b/fs/fat/dir.c
index c4a274285858..249825017da7 100644
--- a/fs/fat/dir.c
+++ b/fs/fat/dir.c
@@ -722,7 +722,7 @@ static int func(struct dir_context *ctx, const char *name, int name_len,   \
 		if (name_len >= sizeof(d1->d_name))			   \
 			name_len = sizeof(d1->d_name) - 1;		   \
 									   \
-		if (put_user(0, d2->d_name)			||	   \
+		if (put_user(0, &d2->d_name[0])			||	   \
 		    put_user(0, &d2->d_reclen)			||	   \
 		    copy_to_user(d1->d_name, name, name_len)	||	   \
 		    put_user(0, d1->d_name + name_len)		||	   \