linux-kernel - Re: [PATCH v3 06/10] perf annotate: Invalidate register states for untracked instructions

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Message-ID: <aODU_uXKl_2ABcN6@google.com>
Date: Sat, 4 Oct 2025 17:04:14 +0900
From: Namhyung Kim <namhyung@...nel.org>
To: Zecheng Li <zecheng@...gle.com>
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 v3 06/10] perf annotate: Invalidate register states for
 untracked instructions

On Wed, Sep 17, 2025 at 07:58:04PM +0000, Zecheng Li wrote:
> When tracking variable types, instructions that modify a pointer value
> in an untracked way can lead to incorrect type propagation. To prevent
> this, invalidate the register state when encountering such instructions.
> 
> This change invalidates pointer types for various arithmetic and bitwise
> operations that current pointer offset tracking doesn't support, like
> imul, shl, and, inc, etc.
> 
> A special case is added for 'xor reg, reg', which is a common idiom for
> zeroing a register. For this, the register state is updated to be a
> constant with a value of 0.
> 
> This could introduce slight regressions if a variable is zeroed and then
> reused. This can be addressed in the future by using all DWARF locations
> for instruction tracking instead of only the first one.
> 
> Signed-off-by: Zecheng Li <zecheng@...gle.com>
> ---
>  tools/perf/arch/x86/annotate/instructions.c | 29 +++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/tools/perf/arch/x86/annotate/instructions.c b/tools/perf/arch/x86/annotate/instructions.c
> index 709c6f7efe82..3c98f72c423f 100644
> --- a/tools/perf/arch/x86/annotate/instructions.c
> +++ b/tools/perf/arch/x86/annotate/instructions.c
> @@ -411,6 +411,35 @@ static void update_insn_state_x86(struct type_state *state,
>  		return;
>  	}
>  
> +	/* Invalidate register states for other ops which may change pointers */
> +	if (has_reg_type(state, dst->reg1) && !dst->mem_ref &&
> +	    dwarf_tag(&state->regs[dst->reg1].type) == DW_TAG_pointer_type) {
> +		if (!strncmp(dl->ins.name, "imul", 4) || !strncmp(dl->ins.name, "mul", 3) ||
> +		    !strncmp(dl->ins.name, "idiv", 4) || !strncmp(dl->ins.name, "div", 3) ||
> +		    !strncmp(dl->ins.name, "shl", 3)  || !strncmp(dl->ins.name, "shr", 3) ||
> +		    !strncmp(dl->ins.name, "sar", 3)  || !strncmp(dl->ins.name, "and", 3) ||
> +		    !strncmp(dl->ins.name, "or", 2)   || !strncmp(dl->ins.name, "neg", 3) ||
> +		    !strncmp(dl->ins.name, "inc", 3)  || !strncmp(dl->ins.name, "dec", 3)) {
> +			pr_debug_dtp("%s [%x] invalidate reg%d\n",
> +						dl->ins.name, insn_offset, dst->reg1);
> +			state->regs[dst->reg1].ok = false;
> +			state->regs[dst->reg1].copied_from = -1;
> +			return;
> +		}
> +
> +		if (!strncmp(dl->ins.name, "xor", 3) && dst->reg1 == src->reg1) {
> +			/* xor reg, reg clears the register */
> +			pr_debug_dtp("xor [%x] clear reg%d\n",
> +				     insn_offset, dst->reg1);
> +
> +			state->regs[dst->reg1].kind = TSR_KIND_CONST;
> +			state->regs[dst->reg1].imm_value = 0;
> +			state->regs[dst->reg1].ok = false;

I think .ok should be 'true'.

Thanks,
Namhyung


> +			state->regs[dst->reg1].copied_from = -1;
> +			return;
> +		}
> +	}
> +
>  	if (strncmp(dl->ins.name, "mov", 3))
>  		return;
>  
> -- 
> 2.51.0.384.g4c02a37b29-goog
>