linux-kernel - Re: [PING] [PATCH v2] arm: ptrace: Fix mask for thumb breakpoint hook

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Date:   Mon, 18 May 2020 17:02:27 +0200
From:   Fredrik Strupe <fredrik@...upe.net>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PING] [PATCH v2] arm: ptrace: Fix mask for thumb breakpoint hook

On 18.05.2020 16:18, Russell King - ARM Linux admin wrote:
> On Mon, May 18, 2020 at 03:12:06PM +0200, Fredrik Strupe wrote:
>> call_undef_hook() in traps.c applies the same instr_mask for both 16-bit
>> and 32-bit thumb instructions. If instr_mask then is only 16 bits wide
>> (0xffff as opposed to 0xffffffff), the first half-word of 32-bit thumb
>> instructions will be masked out. This makes the function match 32-bit
>> thumb instructions where the second half-word is equal to instr_val,
>> regardless of the first half-word.
>>
>> The result in this case is that all undefined 32-bit thumb instructions
>> with the second half-word equal to de01 (udf #1) work as breakpoints
>> and will raise a SIGTRAP instead of a SIGILL, instead of just the one
>> intended 16-bit instruction. An example of such an instruction is
>> eaa0b650, which is unallocated according to Arm ARM and should raise a
>> SIGILL, but instead raises a SIGTRAP.
>
> How can 0xeaa0b650 match 0xde01 when masked with 0xffff ?
>

Sorry, that is a typo; it should say 0xeaa0de01.

For reference, this is similar to the problem with SETEND emulation that
was fixed in commit fc2266011acc in the mainline kernel
(or as discussed here: https://lkml.org/lkml/2020/4/8/274).

Fredrik