linux-kernel - Re: [PATCH v3] riscv: entry: set a0 = -ENOSYS only when syscall != -1

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Date:   Tue, 18 Jul 2023 23:20:27 +0200
From:   Björn Töpel <bjorn@...nel.org>
To:     Celeste Liu <coelacanthushex@...il.com>,
        Palmer Dabbelt <palmer@...osinc.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
        Björn Töpel 
        <bjorn@...osinc.com>, Conor Dooley <conor.dooley@...rochip.com>,
        linux-riscv@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org,
        Celeste Liu <CoelacanthusHex@...il.com>,
        Felix Yan <felixonmars@...hlinux.org>,
        Ruizhe Pan <c141028@...il.com>,
        Shiqi Zhang <shiqi@...c.iscas.ac.cn>
Subject: Re: [PATCH v3] riscv: entry: set a0 = -ENOSYS only when syscall != -1

Celeste Liu <coelacanthushex@...il.com> writes:

> 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 = -ENOSYS should only be executed
> when syscall != -1 In __seccomp_filter, when seccomp rejected this syscall with
> specified errno, they will set a0 to return number as syscall ABI, and then
> 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 = -ENOSYS is always
> executed. It covered a0 set by seccomp, so we always get ENOSYS when match
> seccomp RET_ERRNO rule.
>
> Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
> Reported-by: Felix Yan <felixonmars@...hlinux.org>
> Co-developed-by: Ruizhe Pan <c141028@...il.com>
> Signed-off-by: Ruizhe Pan <c141028@...il.com>
> Co-developed-by: Shiqi Zhang <shiqi@...c.iscas.ac.cn>
> Signed-off-by: Shiqi Zhang <shiqi@...c.iscas.ac.cn>
> Signed-off-by: Celeste Liu <CoelacanthusHex@...il.com>
> Tested-by: Felix Yan <felixonmars@...hlinux.org>

Reviewed-by: Björn Töpel <bjorn@...osinc.com>