linux-kernel - Re: [PATCH for 5.2] rseq/selftests: Fix Thumb mode build failure on arm32

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Date:   Mon, 8 Jul 2019 12:10:18 -0600
From:   shuah <shuah@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Will Deacon <will.deacon@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        stable <stable@...r.kernel.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joel Fernandes <joelaf@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Watson <davejwatson@...com>,
        Andi Kleen <andi@...stfloor.org>,
        linux-kselftest <linux-kselftest@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Chris Lameter <cl@...ux.com>,
        Russell King <linux@....linux.org.uk>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Paul Turner <pjt@...gle.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        rostedt <rostedt@...dmis.org>, Ben Maurer <bmaurer@...com>,
        linux-api <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        carlos <carlos@...hat.com>, Florian Weimer <fweimer@...hat.com>,
        shuah <shuah@...nel.org>
Subject: Re: [PATCH for 5.2] rseq/selftests: Fix Thumb mode build failure on
 arm32

On 7/8/19 11:58 AM, Mathieu Desnoyers wrote:
> ----- On Jun 30, 2019, at 9:56 AM, Mathieu Desnoyers mathieu.desnoyers@...icios.com wrote:
> 
>> Using ".arm .inst" for the arm signature introduces build issues for
>> programs compiled in Thumb mode because the assembler stays in the
>> arm mode for the rest of the inline assembly. Revert to using a ".word"
>> to express the signature as data instead.
>>
>> The choice of signature is a valid trap instruction on arm32 little
>> endian, where both code and data are little endian.
>>
>> ARMv6+ big endian (BE8) generates mixed endianness code vs data:
>> little-endian code and big-endian data. The data value of the signature
>> needs to have its byte order reversed to generate the trap instruction.
>>
>> Prior to ARMv6, -mbig-endian generates big-endian code and data
>> (which match), so the endianness of the data representation of the
>> signature should not be reversed. However, the choice between BE32
>> and BE8 is done by the linker, so we cannot know whether code and
>> data endianness will be mixed before the linker is invoked. So rather
>> than try to play tricks with the linker, the rseq signature is simply
>> data (not a trap instruction) prior to ARMv6 on big endian. This is
>> why the signature is expressed as data (.word) rather than as
>> instruction (.inst) in assembler.
>>
>> Because a ".word" is used to emit the signature, it will be interpreted
>> as a literal pool by a disassembler, not as an actual instruction.
>> Considering that the signature is not meant to be executed except in
>> scenarios where the program execution is completely bogus, this should
>> not be an issue.
> 
> Now that 5.2 is out before this patch has been merged, can we please
> integrate this patch through the kernel selftests or ARM tree so it
> can be merged into the stable 5.2 branch ?
> 

I will apply it to selftests and send it for 5.3-rc1 and mark it for
stable.

thanks,
-- Shuah