Re: [PATCH v6 bpf-next 08/10] bpf: decouple stack_map_get_build_id_offset() from perf_callchain_entry

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2024-08-14 at 11:54 -0700, Andrii Nakryiko wrote:
> Change stack_map_get_build_id_offset() which is used to convert stack
> trace IP addresses into build ID+offset pairs. Right now this function
> accepts an array of u64s as an input, and uses array of
> struct bpf_stack_build_id as an output.
> 
> This is problematic because u64 array is coming from
> perf_callchain_entry, which is (non-sleepable) RCU protected, so once we
> allows sleepable build ID fetching, this all breaks down.
> 
> But its actually pretty easy to make stack_map_get_build_id_offset()
> works with array of struct bpf_stack_build_id as both input and output.
> Which is what this patch is doing, eliminating the dependency on
> perf_callchain_entry. We require caller to fill out
> bpf_stack_build_id.ip fields (all other can be left uninitialized), and
> update in place as we do build ID resolution.
> 
> We make sure to READ_ONCE() and cache locally current IP value as we
> used it in a few places to find matching VMA and so on. Given this data
> is directly accessible and modifiable by user's BPF code, we should make
> sure to have a consistent view of it.
> 
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

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