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Message-Id: <89f9c556-a5c2-4d81-99fd-b6cc8d9957d2@app.fastmail.com>
Date: Tue, 01 Jul 2025 18:14:28 +0200
From: Alex Rønne Petersen <alex@...xrp.com>
To: "Rich Felker" <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH v2] clone: clear the frame pointer in the child process on
relevant ports
On Tue, Jul 1, 2025, at 18:00, Rich Felker wrote:
> On Thu, Dec 12, 2024 at 05:56:04PM +0100, Alex Rønne Petersen wrote:
>> This just mirrors what is done in the start code for the affected ports, as well
>> as what is already done for the three x86 ports.
>>
>> Clearing the frame pointer helps protect FP-based unwinders which have no way of
>> knowing that the FP register should be considered undefined in the child process
>> portion of clone(). In practice, we found this change to be necessary when
>> running the Zig standard library tests under qemu-aarch64_be with musl linked.
>>
>> This version of the patch omits the branch inversion on x86 and powerpc from the
>> previous version, per the discussion there.
>> ---
>> src/thread/aarch64/clone.s | 3 ++-
>> src/thread/arm/clone.s | 3 ++-
>> src/thread/loongarch64/clone.s | 1 +
>> src/thread/m68k/clone.s | 3 ++-
>> src/thread/microblaze/clone.s | 3 ++-
>> src/thread/mips/clone.s | 3 ++-
>> src/thread/mips64/clone.s | 3 ++-
>> src/thread/mipsn32/clone.s | 3 ++-
>> src/thread/or1k/clone.s | 3 ++-
>> 9 files changed, 17 insertions(+), 8 deletions(-)
>>
>> diff --git a/src/thread/aarch64/clone.s b/src/thread/aarch64/clone.s
>> index e3c83395..9ac272bd 100644
>> --- a/src/thread/aarch64/clone.s
>> +++ b/src/thread/aarch64/clone.s
>> @@ -24,7 +24,8 @@ __clone:
>> // parent
>> ret
>> // child
>> -1: ldp x1,x0,[sp],#16
>> +1: mov fp, 0
>> + ldp x1,x0,[sp],#16
>
> The alias fp does not seem to be supported across all assemblers. I'm
> committing a fix that changes this to use the real name x29 unless
> there's any objection and proposed correction.
That seems fine.
> This should be checked on other archs too.
* arm: fp -> r11
* loongarch: $fp -> $r22
* m68k: %fp -> %a6
* mips: $fp -> $30
I'm not sure for which it's actually necessary, but I see no particular harm in just making the changes anyway, just in case.
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