Re: [PATCH] fix fanotify_mark() breakage on big endian 32bit kernel

[Heinrich Schuchardt]

On 04.07.2014 17:12, Helge Deller wrote:
> This patch affects big endian architectures only.
>
> On those with 32bit userspace and 64bit kernel (CONFIG_COMPAT=y) the
> 64bit mask parameter is correctly constructed out of two 32bit values in
> the compat_fanotify_mark() function and then passed as 64bit parameter
> to the fanotify_mark() syscall.
>
> But for the CONFIG_COMPAT=n case (32bit kernel & userspace),
> compat_fanotify_mark() isn't used and the fanotify_mark syscall implementation

I was not able to find a symbol compat_fanotify_mark. Could you, please, 
indicate were this coding is.

> is used directly. In that case the upper and lower 32 bits of the 64bit mask
> parameter is still swapped on big endian machines and thus leads to
> fanotify_mark failing with -EINVAL.
>
> Here is a strace of the same 32bit executable (fanotify01 testcase from LTP):

https://github.com/linux-test-project/ltp
testcases/kernel/syscalls/fanotify/fanotify01.c
I guess.

> On a 64bit kernel it suceeds:
> syscall_322(0, 0, 0x3, 0x3, 0x266c8, 0x1) = 0x3
> syscall_323(0x3, 0x1, 0, 0x3b, 0xffffff9c, 0x266c8) = 0
>
> On a 32bit kernel it fails:
> syscall_322(0, 0, 0x3, 0x3, 0x266c8, 0x1) = 0x3
> syscall_323(0x3, 0x1, 0, 0x3b, 0xffffff9c, 0x266c8) = -1 (errno 22)

The syscall numbers are architecture specific.
Which architecture did you test on?

> Below is the easiest fix for this problem by simply swapping the upper and
> lower 32bit of the 64 bit mask parameter when building a pure 32bit kernel.

The problem you report is architecture specific.
Is fanotify_user.c really the right place for the correction?
Or should the fix be in the "arch" directory?

Best regards

Heinrich Schuchardt

> But on the other side, using __u64 in a syscall API is IMHO wrong.  This may
> easily break 32bit kernel builds, esp. on big endian machines.
>
> The clean solution would probably be to use SYSCALL_DEFINE5() when
> building a 64bit-kernel, and SYSCALL_DEFINE6() for fanotify_mark() when
> building a pure 32bit kernel, something like this:
>
> #ifdef CONFIG_64BIT
> SYSCALL_DEFINE5(fanotify_mark, int, fanotify_fd, unsigned int, flags,
>                                __u64, mask, int, dfd,
>                                const char  __user *, pathname)
> #else
> SYSCALL_DEFINE6(fanotify_mark, int, fanotify_fd, unsigned int, flags,
>                                  __u32, mask0, __u32, mask1, int, dfd,
>                                  const char  __user *, pathname)
> #endif
>
>
> Signed-off-by: Helge Deller <deller@xxxxxx>
> To: Eric Paris <eparis@xxxxxxxxxx>
> Cc: Heinrich Schuchardt <xypron.glpk@xxxxxx>
> Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
>
>
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 3fdc8a3..374261c 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -787,6 +787,10 @@ SYSCALL_DEFINE5(fanotify_mark, int, fanotify_fd, unsigned int, flags,
>   	struct path path;
>   	int ret;
>
> +#if defined(__BIG_ENDIAN) && !defined(CONFIG_64BIT)
> +	mask = (mask << 32) | (mask >> 32);
> +#endif
> +
>   	pr_debug("%s: fanotify_fd=%d flags=%x dfd=%d pathname=%p mask=%llx\n",
>   		 __func__, fanotify_fd, flags, dfd, pathname, mask);

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