Re: [PATCH v3 4/7] alpha/perf: do not report user registers from the entry code

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] alpha/perf: __entry_save_end precedes r16-r18 save in entSys
--

commit ce97475b8e0171fa2ccbfec6b4cebc978a8757a6
Author: Matt Turner <mattst88@gmail.com>

alpha/perf: do not report user registers from the entry code

This commit prevents perf_get_regs_user() from returning an incomplete user
pt_regs frame when a perf sample interrupts kernel exception or syscall
entry before the user registers have finished being saved.

> diff --git a/arch/alpha/kernel/entry.S b/arch/alpha/kernel/entry.S
> index 790389a15c3ce..2de7cdd3a8725 100644
> --- a/arch/alpha/kernel/entry.S
> +++ b/arch/alpha/kernel/entry.S

[ ... ]

> @@ -473,6 +479,8 @@ CFI_END_OSF_FRAME entDbg
>  	.cfi_rel_offset	$gp, 16
>  entSys:
>  	SAVE_ALL
> +	.globl	__entry_save_end
> +__entry_save_end:
>  	ldq	$1, 0($sp)	/* syscall nr from saved r0 */
>  	stq	$1, 8($sp)	/* regs->r1 = shadow syscall nr */

[Severity: Medium]
Should __entry_save_end be placed after $16, $17, and $18 are stored into
SP_OFF+24..40($sp) in entSys?

Unlike non-syscall PALcode entries, PAL_callsys does not save user $16..$18
into the PALcode stack frame, and SAVE_ALL only stores them into
trap_a0..trap_a2 at 160..176($sp). In entSys, pt_regs->r16..r18 are
populated a few instructions after SAVE_ALL while interrupts are still
enabled:

arch/alpha/kernel/entry.S:
entSys:
	SAVE_ALL
	.globl	__entry_save_end
__entry_save_end:
	ldq	$1, 0($sp)	/* syscall nr from saved r0 */
	stq	$1, 8($sp)	/* regs->r1 = shadow syscall nr */

	lda	$8, 0x3fff
	bic	$sp, $8, $8

	stq	$16, SP_OFF+24($sp)
	stq	$17, SP_OFF+32($sp)
	stq	$18, SP_OFF+40($sp)

Since pt_regs_offset[] in arch/alpha/kernel/perf_regs.c maps
PERF_REG_ALPHA_R16..R18 to offsetof(struct pt_regs, r16..r18) rather than
trap_a0..trap_a2, if a perf interrupt lands between __entry_save_end and
stq $18, SP_OFF+40($sp), in_entry_save_code(regs->pc) will return false and
perf_get_regs_user() will expose task_pt_regs(current) before r16, r17, and
r18 have been saved.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261101-alpha-profileme-v3-0-92d511b5a044@gmail.com?part=4