Re: [Patch v6 10/20] perf: Add counted-bounds tracking for perf_branch_stack

[sashiko-bot@kernel.org]

> struct perf_branch_stack uses a trailing flexible array member,
> entries[], whose valid length is tracked by nr. When this structure
> is embedded as a value member in cpu_hw_events, the compiler can treat
> the trailing flexible array as having a fixed zero-length layout under
> -fstrict-flex-arrays=3 and UBSan, which can lead to strict array-bounds
> diagnostics when entries[] is indexed.
> 
> Annotate entries[] with __counted_by(nr) so the compiler and sanitizer
> track the array length from the nr field. Because a structure containing
> a counted flexible array cannot be embedded as a value member, switch
> the per-CPU LBR stack backing storage to a complete __lbr_stack[] array
> and add a helper cpu_lbr_stack() to recover the struct perf_branch_stack*
> view for x86 and powerpc.
> 
> Also initialize the branch-stack count before populating entries[] to
> avoid false-positive bounds checks while the stack is being filled.
> 
> No functional change intended.
> 
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Cc: Sandipan Das <sandipan.das@amd.com>
> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>

Sashiko has reviewed this patch and found no issues.

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