linux-kernel - Re: [PATCH v3] bpftool: Add CET-aware symbol matching for x86

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Message-ID: <a3e42c2d-0a43-4fe7-8be5-96a3dff723d2@kernel.org>
Date: Fri, 27 Jun 2025 12:08:48 +0100
From: Quentin Monnet <qmo@...nel.org>
To: Yuan Chen <chenyuan_fl@....com>, ast@...nel.org
Cc: bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
 Yuan Chen <chenyuan@...inos.cn>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH v3] bpftool: Add CET-aware symbol matching for x86_64
 architectures

Thanks! Next time, please try to add all relevant maintainers as
recipients or in copy of your message when submitting patches. You can
get the list with get_maintainer.pl, try running it on your patch or with
"./scripts/get_maintainer.pl -f tools/bpf/bpftool/link.c"

2025-06-26 15:49 UTC+0800 ~ Yuan Chen <chenyuan_fl@....com>
> From: Yuan Chen <chenyuan@...inos.cn>
> 
> Adjust symbol matching logic to account for Control-flow Enforcement
> Technology (CET) on x86_64 systems. CET prefixes functions with a 4-byte
> 'endbr' instruction, shifting the actual entry point to symbol + 4.
> 
> Signed-off-by: Yuan Chen <chenyuan@...inos.cn>
> ---
>  tools/bpf/bpftool/link.c | 30 ++++++++++++++++++++++++++++--
>  1 file changed, 28 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/bpf/bpftool/link.c b/tools/bpf/bpftool/link.c
> index 03513ffffb79..dfd192b4c5ad 100644
> --- a/tools/bpf/bpftool/link.c
> +++ b/tools/bpf/bpftool/link.c
> @@ -307,8 +307,21 @@ show_kprobe_multi_json(struct bpf_link_info *info, json_writer_t *wtr)
>  		goto error;
>  
>  	for (i = 0; i < dd.sym_count; i++) {
> -		if (dd.sym_mapping[i].address != data[j].addr)
> +		if (dd.sym_mapping[i].address != data[j].addr) {
> +#if defined(__x86_64__) || defined(__amd64__)


I'm not familiar with CET, but from what I read, it's been around since
Tiger Lake processors (2020). Do we have a risk of false positive with
older CPUs? Maybe check that the instruction at
dd.sym_mapping[i].address is endbr32 or endbr34?


> +			/*
> +			 * On x86_64 architectures with CET (Control-flow Enforcement Technology),
> +			 * function entry points have a 4-byte 'endbr' instruction prefix.
> +			 * This causes the actual function address = symbol address + 4.
> +			 * Here we check if this symbol matches the target address minus 4,
> +			 * indicating we've found a CET-enabled function entry point.
> +			 */
> +			if (dd.sym_mapping[i].address == data[j].addr - 4)
> +				goto found;
> +#endif
>  			continue;
> +		}
> +found:
>  		jsonw_start_object(json_wtr);
>  		jsonw_uint_field(json_wtr, "addr", dd.sym_mapping[i].address);


I suppose we still want to print dd.sym_mapping[i].address (and not
data[j].addr) when we found it with the CET offset here - just
double-checking.


>  		jsonw_string_field(json_wtr, "func", dd.sym_mapping[i].name);
> @@ -744,8 +757,21 @@ static void show_kprobe_multi_plain(struct bpf_link_info *info)
>  
>  	printf("\n\t%-16s %-16s %s", "addr", "cookie", "func [module]");
>  	for (i = 0; i < dd.sym_count; i++) {
> -		if (dd.sym_mapping[i].address != data[j].addr)
> +		if (dd.sym_mapping[i].address != data[j].addr) {
> +#if defined(__x86_64__) || defined(__amd64__)
> +			/*
> +			 * On x86_64 architectures with CET (Control-flow Enforcement Technology),
> +			 * function entry points have a 4-byte 'endbr' instruction prefix.
> +			 * This causes the actual function address = symbol address + 4.
> +			 * Here we check if this symbol matches the target address minus 4,
> +			 * indicating we've found a CET-enabled function entry point.
> +			 */
> +			if (dd.sym_mapping[i].address == data[j].addr - 4)
> +				goto found;
> +#endif


Given that we have twice the same check, I'd move this to a dedicated
wrapper function that we could call from both show_kprobe_multi_json()
and show_kprobe_multi_plain().


>  			continue;
> +		}
> +found:
>  		printf("\n\t%016lx %-16llx %s",
>  		       dd.sym_mapping[i].address, data[j].cookie, dd.sym_mapping[i].name);
>  		if (dd.sym_mapping[i].module[0] != '\0')