Re: [PATCH v2] riscv: entry: always initialize regs->a0 to -ENOSYS

[Andrea Bolognani]

On Thu, Jun 27, 2024 at 10:23:39PM GMT, Celeste Liu wrote:
> Otherwise when the tracer changes syscall number to -1, the kernel fails
> to initialize a0 with -ENOSYS and subsequently fails to return the error
> code of the failed syscall to userspace. For example, it will break
> strace syscall tampering.
>
> Fixes: 52449c17bdd1 ("riscv: entry: set a0 = -ENOSYS only when syscall != -1")
> Reported-by: "Dmitry V. Levin" <ldv@xxxxxxxxx>
> Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Celeste Liu <CoelacanthusHex@xxxxxxxxx>
> ---
>  arch/riscv/kernel/traps.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index 05a16b1f0aee..51ebfd23e007 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -319,6 +319,7 @@ void do_trap_ecall_u(struct pt_regs *regs)
>
>  		regs->epc += 4;
>  		regs->orig_a0 = regs->a0;
> +		regs->a0 = -ENOSYS;
>
>  		riscv_v_vstate_discard(regs);
>
> @@ -328,8 +329,7 @@ void do_trap_ecall_u(struct pt_regs *regs)
>
>  		if (syscall >= 0 && syscall < NR_syscalls)
>  			syscall_handler(regs, syscall);
> -		else if (syscall != -1)
> -			regs->a0 = -ENOSYS;
> +
>  		/*
>  		 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
>  		 * so the maximum stack offset is 1k bytes (10 bits).

Hi,

this change seems to have broken strace's test suite.

In particular, the "legacy_syscall_info" test, which is meant to
verify that strace behaves correctly when PTRACE_GET_SYSCALL_INFO is
not available, reports a bogus value for the first argument of the
syscall (the one passed via a0).

The bogus value comes directly from the ptrace() call, before strace
has a chance to meddle with it, hence why the maintainer suggested
that the issue would likely be traced back to the kernel.

I have built a kernel with this change reverted and, as expected, the
strace test suite passes. Admittedly I've used the 6.11-rc7 Fedora
kernel as the baseline for this test, but none of the Fedora patches
touch the RISC-V code at all and the file itself hasn't been touched
since rc7, so I'm fairly confident the same behavior is present in
vanilla 6.11 too.

See

  https://github.com/strace/strace/issues/315

for the original report. Please let me know if I need to provide
additional information, report this anywhere else (bugzilla?), and so
on...

Thanks!

-- 
Andrea Bolognani / Red Hat / Virtualization