linux-kernel - Re: [PATCH v2 07/10] perf annotate: Invalidate register states for unsupported instructions

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Message-ID: <CAJUgMy++zvbgcuzyQUiVHdaTZMa8o=LtSUzy9PXe_sqXHqK7nQ@mail.gmail.com>
Date: Wed, 3 Sep 2025 16:54:39 -0400
From: Zecheng Li <zecheng@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	"Liang, Kan" <kan.liang@...ux.intel.com>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Xu Liu <xliuprof@...gle.com>, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 07/10] perf annotate: Invalidate register states for
 unsupported instructions

On Sat, Aug 30, 2025 at 3:15 AM Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Mon, Aug 25, 2025 at 07:57:48PM +0000, Zecheng Li wrote:
> > Invalidate register states when encountering unsupported instructions
> > that modify pointers, to prevent propagating incorrect pointer types.
> >
> > On x86, the 'xor' instruction may appear in a predecessor basic block
> > and zero out a register that invalidates the target register state. This
> > sometimes relates to tagged pointers and normal programs should not
> > dereference NULL pointers, so we assume such execution paths are invalid
> > and do not invalidate states for 'xor' instructions.
>
> Probably we can set it to 0 with TSR_KIND_CONST.
>
It seems TSR_KIND_CONST doesn't relate to a type. Although the value
was set to 0, it still has the pointer type. I see regressions that
has this pattern

xorl    %rax, %rax
mov    %rax, (%rsp)

and sometimes

xorl    %rax, %rax
... (some branches)
mov    (%rax), %rbx

Normally NULL pointer dereference should not happen, so I assume such
execution paths are invalid.