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References: <20230718210037.250665-1-CoelacanthusHex@gmail.com>
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From:   Guo Ren <guoren@kernel.org>
Date:   Wed, 19 Jul 2023 07:40:45 +0800
Message-ID: <CAJF2gTRf0UOYhqwUm6=1YMvZt8q_72WRUHqN=L+nOWaipL+H1Q@mail.gmail.com>
Subject: Re: [PATCH v3] riscv: entry: set a0 = -ENOSYS only when syscall != -1
To:     Celeste Liu <coelacanthushex@gmail.com>,
        Huacai Chen <chenhuacai@kernel.org>
Cc:     Palmer Dabbelt <palmer@rivosinc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@rivosinc.com>,
        Conor Dooley <conor.dooley@microchip.com>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        Felix Yan <felixonmars@archlinux.org>,
        Ruizhe Pan <c141028@gmail.com>,
        Shiqi Zhang <shiqi@isrc.iscas.ac.cn>
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On Wed, Jul 19, 2023 at 5:01=E2=80=AFAM Celeste Liu <coelacanthushex@gmail.=
com> wrote:
>
> When we test seccomp with 6.4 kernel, we found errno has wrong value.
> If we deny NETLINK_AUDIT with EAFNOSUPPORT, after f0bddf50586d, we will
> get ENOSYS instead. We got same result with commit 9c2598d43510 ("riscv: =
entry:
> Save a0 prior syscall_enter_from_user_mode()").
>
> After analysing code, we think that regs->a0 =3D -ENOSYS should only be e=
xecuted
> when syscall !=3D -1 In __seccomp_filter, when seccomp rejected this sysc=
all with
> specified errno, they will set a0 to return number as syscall ABI, and th=
en
> return -1. This return number is finally pass as return number of
> syscall_enter_from_user_mode, and then is compared with NR_syscalls after
> converted to ulong (so it will be ULONG_MAX). The condition
> syscall < NR_syscalls will always be false, so regs->a0 =3D -ENOSYS is al=
ways
> executed. It covered a0 set by seccomp, so we always get ENOSYS when matc=
h
> seccomp RET_ERRNO rule.
>
> Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
> Reported-by: Felix Yan <felixonmars@archlinux.org>
> Co-developed-by: Ruizhe Pan <c141028@gmail.com>
> Signed-off-by: Ruizhe Pan <c141028@gmail.com>
> Co-developed-by: Shiqi Zhang <shiqi@isrc.iscas.ac.cn>
> Signed-off-by: Shiqi Zhang <shiqi@isrc.iscas.ac.cn>
> Signed-off-by: Celeste Liu <CoelacanthusHex@gmail.com>
> Tested-by: Felix Yan <felixonmars@archlinux.org>
> ---
>
> v2 -> v3: use if-statement instead of set default value,
>           clarify the type of syscall
> v1 -> v2: added explanation on why always got ENOSYS
>
>  arch/riscv/kernel/traps.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index f910dfccbf5d2..5cef728745420 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -297,6 +297,10 @@ asmlinkage __visible __trap_section void do_trap_bre=
ak(struct pt_regs *regs)
>  asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs =
*regs)
>  {
>         if (user_mode(regs)) {
> +               /*
> +                * Convert negative numbers to very high and thus out of =
range
> +                * numbers for comparisons.
> +                */
>                 ulong syscall =3D regs->a7;
>
>                 regs->epc +=3D 4;
> @@ -308,7 +312,7 @@ asmlinkage __visible __trap_section void do_trap_ecal=
l_u(struct pt_regs *regs)
>
>                 if (syscall < NR_syscalls)
>                         syscall_handler(regs, syscall);
> -               else
> +               else if ((long)syscall !=3D -1L)
Maybe we should define an explicit macro for this ERRNO in
__seccomp_filter, and this style obeys the coding convention.

For this patch:
Reviewed-by: Guo Ren <guoren@kernel.org>

Cc: loongarch guy, please check loongarch's code. :)

>                         regs->a0 =3D -ENOSYS;
>
>                 syscall_exit_to_user_mode(regs);
> --
> 2.41.0
>


--=20
Best Regards
 Guo Ren