Re: [PATCH bpf-next 6/6] selftests/bpf: Add arena fault tests for atomics with fetch

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2026-08-10 at 15:43 +0200, Daniel Borkmann wrote:
> Add stream_arena_xchg_fault and stream_arena_cmpxchg_fault next to the
> existing read, write and load-acquire fault tests, covering the two
> places a read-modify-write can deposit the old value: src_reg for a
> BPF_XCHG and r0 for a BPF_CMPXCHG. Both cover both halves of the JIT
> bug that left the fetch destination alone when a RMW on an arena pointer
> faulted:
> 
>   - the fault has to be reported as a WRITE, and at the address held by
>     the destination register, which __stderr() and test_address() check
>   - the register receiving the fetched value has to be cleared by the
>     fault handler, which the programs check by poisoning it before the
>     atomic and returning it, so __retval(0) fails if it is left untouched
> 
> Note, the atomics are open coded since linux/filter.h cannot be included
> alongside vmlinux.h.
> 
>   # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t stream
>   [...]
>   #464/1   stream_arena_fault_address/read_fault:OK
>   #464/2   stream_arena_fault_address/write_fault:OK
>   #464/3   stream_arena_fault_address/load_acquire_fault:OK
>   #464/4   stream_arena_fault_address/xchg_fault:OK
>   #464/5   stream_arena_fault_address/cmpxchg_fault:OK
>   #464     stream_arena_fault_address:OK
>   [...]
>   #466/5   stream_success/stream_arena_write_fault:OK
>   #466/6   stream_success/stream_arena_read_fault:OK
>   #466/7   stream_success/stream_arena_load_acquire_fault:OK
>   #466/8   stream_success/stream_arena_xchg_fault:OK
>   #466/9   stream_success/stream_arena_cmpxchg_fault:OK
>   [...]
>   Summary: 4/22 PASSED, 0 SKIPPED, 0 FAILED
> 
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> ---

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

...

> diff --git a/tools/testing/selftests/bpf/prog_tests/stream.c b/tools/testing/selftests/bpf/prog_tests/stream.c
> index 15dd3ae2a84b..e4e9374309e2 100644
> --- a/tools/testing/selftests/bpf/prog_tests/stream.c
> +++ b/tools/testing/selftests/bpf/prog_tests/stream.c
> @@ -105,6 +105,10 @@ void test_stream_arena_fault_address(void)
>  		test_address(skel->progs.stream_arena_write_fault, &skel->bss->fault_addr);
>  	if (test__start_subtest("load_acquire_fault"))
>  		test_address(skel->progs.stream_arena_load_acquire_fault, &skel->bss->fault_addr);
> +	if (test__start_subtest("xchg_fault"))
> +		test_address(skel->progs.stream_arena_xchg_fault, &skel->bss->fault_addr);
> +	if (test__start_subtest("cmpxchg_fault"))
> +		test_address(skel->progs.stream_arena_cmpxchg_fault, &skel->bss->fault_addr);

Tbh, this seem to be an overkill. The point of these stream tests is
to check the stream output format details etc. Here the __stderr
annotation in the test already captures that the fault had happened
and also checks retval.

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