linux-kernel - Re: [PATCH] riscv/entry: get correct syscall number from syscall_get

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Message-ID: <87sesgftng.fsf@all.your.base.are.belong.to.us>
Date: Mon, 28 Oct 2024 02:45:07 -0700
From: Björn Töpel <bjorn@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>, Celeste Liu
 <coelacanthushex@...il.com>, Celeste Liu via B4 Relay
 <devnull+CoelacanthusHex.gmail.com@...nel.org>, Paul Walmsley
 <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou
 <aou@...s.berkeley.edu>, Björn Töpel
 <bjorn@...osinc.com>
Cc: Palmer Dabbelt <palmer@...osinc.com>, Alexandre Ghiti <alex@...ti.fr>,
 "Dmitry V. Levin" <ldv@...ace.io>, Andrea Bolognani <abologna@...hat.com>,
 Felix Yan <felixonmars@...hlinux.org>, Ruizhe Pan <c141028@...il.com>,
 Shiqi Zhang <shiqi@...c.iscas.ac.cn>, Guo Ren <guoren@...nel.org>, Yao Zi
 <ziyao@...root.org>, Han Gao <gaohan@...as.ac.cn>,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH] riscv/entry: get correct syscall number from
 syscall_get_nr()

Thanks for helping out to dissect this! Much appreciated!

Thomas Gleixner <tglx@...utronix.de> writes:

> Let me look at your failure analysis from your first reply:
>
>>  1. strace "tracing": Requires that regs->a0 is not tampered with prior
>>     ptrace notification
>> 
>>     E.g.:
>>     | # ./strace /
>>     | execve("/", ["/"], 0x7ffffaac3890 /* 21 vars */) = -1 EACCES (Permission denied)
>>     | ./strace: exec: Permission denied
>>     | +++ exited with 1 +++
>>     | # ./disable_ptrace_get_syscall_info ./strace /
>>     | execve(0xffffffffffffffda, ["/"], 0x7fffd893ce10 /* 21 vars */) = -1 EACCES (Permission denied)
>>     | ./strace: exec: Permission denied
>>     | +++ exited with 1 +++
>> 
>>     In the second case, arg0 is prematurely set to -ENOSYS
>>     (0xffffffffffffffda).
>
> That's expected if ptrace_get_syscall_info() is not used. Plain dumping
> registers will give you the current value on all architectures.
> ptrace_get_syscall_info() exist exactly for that reason.

Noted! So this shouldn't be considered as a regression. IOW, the
existing upstream code is OK for this scenario.

>>  2. strace "syscall tampering": Requires that ENOSYS is returned for
>>     syscall(-1), and not skipped w/o a proper return value.
>> 
>>     E.g.:
>>     | ./strace -a0 -ewrite -einject=write:error=enospc echo helloject=write:error=enospc echo hello   
>> 
>>     Here, strace expects that injecting -1, would result in a ENOSYS.
>
> No. It expects ENOSPC with the above command line. man strace:
>
>        If :error=errno option is specified, a fault is injected into a
>        syscall invocation: the syscall number is replaced by -1 which
>        corresponds to an invalid syscall (unless a syscall is specified
>        with :syscall= option), and the error code is specified using a
>        symbolic errno value like ENOSYS or a numeric value within
>        1..4095 range.
>
> Similar for -einject:retval=$N
>
> So you cannot overwrite a0 with ENOSYS if the syscall needs to be
> skipped.

ACK!