Re: [PATCH] perf machine: Fix NULL deref in machine__is_lock_function()

[sashiko-bot@kernel.org]

> machine__is_lock_function() looks up __sched_text_start and, if found,
> assumes the remaining scheduler/lock text symbols are present too: it
> dereferences sym->start for __sched_text_end, __lock_text_start and
> __lock_text_end without a NULL check, under a "should not fail from
> here" comment.
> 
> That assumption has already been broken in practice: on s390,
> __sched_text_end shared its address with __cpuidle_text_start and
> was dropped by symbols__fixup_duplicate(), causing a SIGSEGV in
> "perf lock contention".
> commit d8d85ce86dc8 ("perf lock contention: Fix core dump related to
> not finding the "__sched_text_end" symbol on s/390") worked around
> that particular case by keeping duplicate symbols via
> symbol_conf.allow_aliases. The symbols can also be absent when the
> kernel image has no lock text, or when the symbol table is otherwise
> incomplete.
> 
> NULL-check each of the four lookups and share the existing failure
> handling through a fail label that records the "failed to get kernel
> symbols" state (machine->sched.text_start = 1), so the initialization
> is only cached as complete when every lookup succeeded and later calls
> bail out early without retrying the symbol lookups.
> 
> Fixes: 0d2997f750d1 ("perf lock: Look up callchain for the contended locks")
> Signed-off-by: Gu Bin <gubin@cmss.chinamobile.com>

Sashiko has reviewed this patch and found no issues.

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