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Date: Tue, 21 Jul 2026 17:04:34 +0200
From: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
To: Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= <philmd@oss.qualcomm.com>
Cc: Oleg Nesterov <oleg@redhat.com>, James Hogan <jhogan@kernel.org>,
	Kees Cook <kees@kernel.org>, linux-mips@vger.kernel.org,
	linux-kernel@vger.kernel.org, Jiaxun Yang <jiaxun.yang@flygoat.com>
Subject: Re: [PATCH] MIPS: ptrace: Fix syscall skipping via PTRACE_SYSCALL
Message-ID: <al-KglT33I2jzUC7@alpha.franken.de>
References: <20260717112731.59254-1-tsbogend@alpha.franken.de>
 <27072da0-124d-4a87-b037-f56320c8e97b@oss.qualcomm.com>
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In-Reply-To: <27072da0-124d-4a87-b037-f56320c8e97b@oss.qualcomm.com>

On Tue, Jul 21, 2026 at 04:43:40PM +0200, Philippe Mathieu-Daudé wrote:
> Hi Thomas,
> 
> +Jiaxun
> 
> On 17/7/26 13:27, Thomas Bogendoerfer wrote:
> > If tracer wanted to skip a syscall return value was always
> > overwritten with -ENOSYS. Fix this by checking against original
> > syscall number and only return -ENOSYS, if it is negative.
> > 
> > Fixes: b6318a903d06 ("MIPS/ptrace: Pick up ptrace/seccomp changed syscalls")
> 
> I think you meant:
> 
> Fixes: 4370b673ccf2 ("MIPS: scall: Save thread_info.syscall unconditionally
> on entry")

no it was already broken before that commit.

b6318a903d06 reloads syscall number after tracehook_report_syscall_entry() so
the check for negative syscall is done with the "manipulated" syscall number.
And it the syscall should be skipped that number is -1.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]