linux-kernel - Re: [tip: x86/core] x86/alternative: Patch a single alternative location only once

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Message-ID: <ddb38d36-0194-414c-8614-1f37b1f38283@suse.com>
Date: Tue, 14 Oct 2025 16:13:11 +0200
From: Jürgen Groß <jgross@...e.com>
To: Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org
Cc: linux-tip-commits@...r.kernel.org,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org
Subject: Re: [tip: x86/core] x86/alternative: Patch a single alternative
 location only once

On 14.10.25 15:25, Borislav Petkov wrote:
> On Tue, Oct 14, 2025 at 02:59:09PM +0200, Borislav Petkov wrote:
>> And frankly, the justification for this patch is also meh: an interrupt might
>> use the location?!? If this is a real issue then we better disable IRQs around
>> it. But not make the code yucky.
> 
> And this patch is not doing what it is claiming is doing:
> 
> Here's an incarnation of XSTATE_XSAVE which is a 3-way alternative:
> 
> At location ffffffff81283cd3, it replaces the default XSAVE with XSAVEOPT.
> 
> [    2.456696] SMP alternatives: feat: 10*32+0, old: (save_fpregs_to_fpstate+0x43/0xa0 (ffffffff81283cd3) len: 6), repl: (ffffffff89c1e6d9, len: 6) flags: 0x0
> [    2.459317] SMP alternatives: ffffffff81283cd3:   old_insn: 49 0f ae 64 24 40	xsave64 0x40(%r12)
> [    2.460806] SMP alternatives: ffffffff89c1e6d9:   rpl_insn: 49 0f ae 74 24 40	xsaveopt64 0x40(%r12)
> [    2.463316] SMP alternatives: ffffffff81283cd3: final_insn: 49 0f ae 74 24 40
> 
> and then that exact same location:
> 
> [    2.464757] SMP alternatives: feat: 10*32+1, old: (save_fpregs_to_fpstate+0x43/0xa0 (ffffffff81283cd3) len: 6), repl: (ffffffff89c1e6df, len: 6) flags: 0x0
> [    2.467317] SMP alternatives: ffffffff81283cd3:   old_insn: 49 0f ae 64 24 40
> [    2.468746] SMP alternatives: ffffffff89c1e6df:   rpl_insn: 49 0f c7 64 24 40	xsaves64 0x40(%r12)
> [    2.470167] SMP alternatives: ffffffff81283cd3: final_insn: 49 0f c7 64 24 40
> 
> gets XSAVES.

Oh, indeed, I should have adapted the printing. The patching is fine,
it is just the debug output which claims to have written 2 different
instructions to old_insn, while it didn't.

> 
> So, long story short, this needs more thought:
> 
> 1. check whether patching is needed
> 
> 2. a helper function evaluates all instances and figures out the final insn
>     bytes which need to be patched along with the proper padding
> 
> 3. the proper bytes are copied into the target location and all good
> 
> But not like this - the idea is somewhat ok but it needs to be executed in
> a cleaner manner.
> 
> I'd say.

I can have a try with this approach.

Could take some time, vacation is coming up...


Juergen

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