Re: [PATCH 09/15] sh: rework sync_file_range ABI

[John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>]

Hi Arnd,

thanks for your patch!

On Thu, 2024-06-20 at 18:23 +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> The unusual function calling conventions on superh ended up causing
                                              ^^^^^^
                                       It's spelled SuperH

> sync_file_range to have the wrong argument order, with the 'flags'
> argument getting sorted before 'nbytes' by the compiler.
> 
> In userspace, I found that musl, glibc, uclibc and strace all expect the
> normal calling conventions with 'nbytes' last, so changing the kernel
> to match them should make all of those work.
> 
> In order to be able to also fix libc implementations to work with existing
> kernels, they need to be able to tell which ABI is used. An easy way
> to do this is to add yet another system call using the sync_file_range2
> ABI that works the same on all architectures.
> 
> Old user binaries can now work on new kernels, and new binaries can
> try the new sync_file_range2() to work with new kernels or fall back
> to the old sync_file_range() version if that doesn't exist.
> 
> Cc: stable@vger.kernel.org
> Fixes: 75c92acdd5b1 ("sh: Wire up new syscalls.")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  arch/sh/kernel/sys_sh32.c           | 11 +++++++++++
>  arch/sh/kernel/syscalls/syscall.tbl |  3 ++-
>  2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/sh/kernel/sys_sh32.c b/arch/sh/kernel/sys_sh32.c
> index 9dca568509a5..d5a4f7c697d8 100644
> --- a/arch/sh/kernel/sys_sh32.c
> +++ b/arch/sh/kernel/sys_sh32.c
> @@ -59,3 +59,14 @@ asmlinkage int sys_fadvise64_64_wrapper(int fd, u32 offset0, u32 offset1,
>  				 (u64)len0 << 32 | len1, advice);
>  #endif
>  }
> +
> +/*
> + * swap the arguments the way that libc wants it instead of

I think "swap the arguments to the order that libc wants them" would
be easier to understand here.

> + * moving flags ahead of the 64-bit nbytes argument
> + */
> +SYSCALL_DEFINE6(sh_sync_file_range6, int, fd, SC_ARG64(offset),
> +                SC_ARG64(nbytes), unsigned int, flags)
> +{
> +        return ksys_sync_file_range(fd, SC_VAL64(loff_t, offset),
> +                                    SC_VAL64(loff_t, nbytes), flags);
> +}
> diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
> index bbf83a2db986..c55fd7696d40 100644
> --- a/arch/sh/kernel/syscalls/syscall.tbl
> +++ b/arch/sh/kernel/syscalls/syscall.tbl
> @@ -321,7 +321,7 @@
>  311	common	set_robust_list			sys_set_robust_list
>  312	common	get_robust_list			sys_get_robust_list
>  313	common	splice				sys_splice
> -314	common	sync_file_range			sys_sync_file_range
> +314	common	sync_file_range			sys_sh_sync_file_range6
                                                                 ^^^^^^ Why the suffix 6 here?

>  315	common	tee				sys_tee
>  316	common	vmsplice			sys_vmsplice
>  317	common	move_pages			sys_move_pages
> @@ -395,6 +395,7 @@
>  385	common	pkey_alloc			sys_pkey_alloc
>  386	common	pkey_free			sys_pkey_free
>  387	common	rseq				sys_rseq
> +388	common	sync_file_range2		sys_sync_file_range2
>  # room for arch specific syscalls
>  393	common	semget				sys_semget
>  394	common	semctl				sys_semctl

I wonder how you discovered this bug. Did you look up the calling convention on SuperH
and compare the argument order for the sys_sync_file_range system call documented there
with the order in the kernel?

Did you also check what order libc uses? I would expect libc on SuperH misordering the
arguments as well unless I am missing something. Or do we know that the code is actually
currently broken?

Thanks,
Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer
`. `'   Physicist
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