linux-kernel - Re: [PATCH v3 02/10] perf/amd/ibs: Remove pointless sample period check

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20241218133533.GD2354@noisy.programming.kicks-ass.net>
Date: Wed, 18 Dec 2024 14:35:33 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Ravi Bangoria <ravi.bangoria@....com>
Cc: mingo@...hat.com, namhyung@...nel.org, acme@...nel.org,
	eranian@...gle.com, mark.rutland@....com,
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
	irogers@...gle.com, adrian.hunter@...el.com,
	kan.liang@...ux.intel.com, tglx@...utronix.de, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	santosh.shukla@....com, ananth.narayan@....com,
	sandipan.das@....com
Subject: Re: [PATCH v3 02/10] perf/amd/ibs: Remove pointless sample period
 check

On Tue, Dec 10, 2024 at 09:34:41AM +0000, Ravi Bangoria wrote:
> Valid perf event sample period value for IBS PMUs (Fetch and Op both)
> is limited to multiple of 0x10. perf_ibs_init() has this check:
> 
>   if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
>           return -EINVAL;
> 
> But it's broken since hwc->sample_period will always be 0 when
> event->attr.sample_freq is 0 (irrespective of event->attr.freq value.)
> 
> One option to fix this is to change the condition:
> 
>   - if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
>   + if (!event->attr.freq && hwc->sample_period & 0x0f)
> 
> However, that will break all userspace tools which have been using IBS
> event with sample_period not multiple of 0x10.
> 
> Another option is to remove the condition altogether and mask lower
> nibble _silently_, same as what current code is inadvertently doing.
> I'm preferring this approach as it keeps the existing behavior.

Urgh... :/

> Acked-by: Namhyung Kim <namhyung@...nel.org>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@....com>
> ---
>  arch/x86/events/amd/ibs.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
> index 4ca8006d2221..bd8919e7c3b1 100644
> --- a/arch/x86/events/amd/ibs.c
> +++ b/arch/x86/events/amd/ibs.c
> @@ -307,13 +307,8 @@ static int perf_ibs_init(struct perf_event *event)
>  		if (config & perf_ibs->cnt_mask)
>  			/* raw max_cnt may not be set */
>  			return -EINVAL;
> -		if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
> -			/*
> -			 * lower 4 bits can not be set in ibs max cnt,
> -			 * but allowing it in case we adjust the
> -			 * sample period to set a frequency.
> -			 */
> -			return -EINVAL;
> +
> +		/* Silently mask off lower nibble. IBS hw mandates it. */
>  		hwc->sample_period &= ~0x0FULL;
>  		if (!hwc->sample_period)
>  			hwc->sample_period = 0x10;
> -- 
> 2.43.0
>