[PATCH] riscv: Avoid fortify warning in syscall_get_arguments()

[Palmer Dabbelt]

On Wed, 09 Apr 2025 14:24:46 PDT (-0700), nathan at kernel.org wrote:
> When building with CONFIG_FORTIFY_SOURCE=y and W=1, there is a warning
> because of the memcpy() in syscall_get_arguments():
>
>   In file included from include/linux/string.h:392,
>                    from include/linux/bitmap.h:13,
>                    from include/linux/cpumask.h:12,
>                    from arch/riscv/include/asm/processor.h:55,
>                    from include/linux/sched.h:13,
>                    from kernel/ptrace.c:13:
>   In function 'fortify_memcpy_chk',
>       inlined from 'syscall_get_arguments.isra' at arch/riscv/include/asm/syscall.h:66:2:
>   include/linux/fortify-string.h:580:25: error: call to '__read_overflow2_field' declared with attribute warning: detected read beyond size of field (2nd parameter); maybe use struct_group()? [-Werror=attribute-warning]
>     580 |                         __read_overflow2_field(q_size_field, size);
>         |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   cc1: all warnings being treated as errors
>
> The fortified memcpy() routine enforces that the source is not overread
> and the destination is not overwritten if the size of either field and
> the size of the copy are known at compile time. The memcpy() in
> syscall_get_arguments() intentionally overreads from a1 to a5 in
> 'struct pt_regs' but this is bigger than the size of a1.
>
> Normally, this could be solved by wrapping a1 through a5 with
> struct_group() but there was already a struct_group() applied to these
> members in commit bba547810c66 ("riscv: tracing: Fix
> __write_overflow_field in ftrace_partial_regs()").
>
> Just avoid memcpy() altogether and write the copying of args from regs
> manually, which clears up the warning at the expense of three extra
> lines of code.

You could still memcpy, but you'd need some sort of

    memcpy(args, &regs->aregs + sizeof(args[0]), 5 * sizeof(args[0]));

(or however you index a struct group).  I think it's saner to just do it 
with the manual copies, though, as I'd have to look up what this code 
does every time I run into it.

> Signed-off-by: Nathan Chancellor <nathan at kernel.org>
> ---
> I omitted a Fixes tag because I think this has always been an overread
> if I understand correctly but it is only the addition of the checks from
> commit f68f2ff91512 ("fortify: Detect struct member overflows in
> memcpy() at compile-time") that it becomes a noticeable issue.

I'm going to add the Fixes.  It's a suprious warning (there's no actual 
crash from the overread), but even spurious warnings are a headache for 
peolpe trying to build stuff.

> This came out of a discussion from the addition of
> syscall_set_arguments(), where the same logic causes a more noticeable
> fortify warning because it happens without W=1, as it is an overwrite:
> https://lore.kernel.org/20250408213131.GA2872426@ax162/
> ---
>  arch/riscv/include/asm/syscall.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/syscall.h b/arch/riscv/include/asm/syscall.h
> index 121fff429dce66b31fe79b691b8edd816c8019e9..eceabf59ae482aa1832b09371ddb3ba8cd65f91d 100644
> --- a/arch/riscv/include/asm/syscall.h
> +++ b/arch/riscv/include/asm/syscall.h
> @@ -62,8 +62,11 @@ static inline void syscall_get_arguments(struct task_struct *task,
>  					 unsigned long *args)
>  {
>  	args[0] = regs->orig_a0;
> -	args++;
> -	memcpy(args, &regs->a1, 5 * sizeof(args[0]));
> +	args[1] = regs->a1;
> +	args[2] = regs->a2;
> +	args[3] = regs->a3;
> +	args[4] = regs->a4;
> +	args[5] = regs->a5;
>  }
>
>  static inline int syscall_get_arch(struct task_struct *task)
>
> ---
> base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
> change-id: 20250409-riscv-avoid-fortify-warning-syscall_get_arguments-19c0495d4ed7
>
> Best regards,