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References: <20230720140348.4716-1-CoelacanthusHex@gmail.com>
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From:   Emil Renner Berthing <emil.renner.berthing@canonical.com>
Date:   Fri, 21 Jul 2023 18:17:30 +0200
Message-ID: <CAJM55Z83u+aEVYAA0+9PEXK6hbre2=FS_=SDdxQ1uQN7L2PjkA@mail.gmail.com>
Subject: Re: [PATCH v4] riscv: entry: set a0 = -ENOSYS only when syscall != -1
To:     Celeste Liu <coelacanthushex@gmail.com>
Cc:     Palmer Dabbelt <palmer@rivosinc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>, Guo Ren <guoren@kernel.org>,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@rivosinc.com>,
        Conor Dooley <conor.dooley@microchip.com>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        Andreas Schwab <schwab@suse.de>,
        David Laight <David.Laight@aculab.com>,
        Felix Yan <felixonmars@archlinux.org>,
        Ruizhe Pan <c141028@gmail.com>,
        Shiqi Zhang <shiqi@isrc.iscas.ac.cn>
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On Thu, 20 Jul 2023 at 16:05, 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>

With this patch I can use groupmod, groupadd, usermod, useradd etc.
inside systemd-nspawn again. Thanks!
Tested-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>

> Reviewed-by: Bj=C3=B6rn T=C3=B6pel <bjorn@rivosinc.com>
> Reviewed-by: Guo Ren <guoren@kernel.org>
> ---
>
> v3 -> v4: use long instead of ulong to reduce type cast and avoid
>           implementation-defined behavior, and make the judgment of sysca=
ll
>           invalid more explicit
> 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, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index f910dfccbf5d2..729f79c97e2bf 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -297,7 +297,7 @@ asmlinkage __visible __trap_section void do_trap_brea=
k(struct pt_regs *regs)
>  asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs =
*regs)
>  {
>         if (user_mode(regs)) {
> -               ulong syscall =3D regs->a7;
> +               long syscall =3D regs->a7;
>
>                 regs->epc +=3D 4;
>                 regs->orig_a0 =3D regs->a0;
> @@ -306,9 +306,9 @@ asmlinkage __visible __trap_section void do_trap_ecal=
l_u(struct pt_regs *regs)
>
>                 syscall =3D syscall_enter_from_user_mode(regs, syscall);
>
> -               if (syscall < NR_syscalls)
> +               if (syscall >=3D 0 && syscall < NR_syscalls)
>                         syscall_handler(regs, syscall);
> -               else
> +               else if (syscall !=3D -1)
>                         regs->a0 =3D -ENOSYS;
>
>                 syscall_exit_to_user_mode(regs);
> --
> 2.41.0
>
>
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