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selftests/bpf: libarena: Fix can-loop zero variable definition

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BPF can_loop based loops require the index variable to stay imprecise.
This means we must initialize them from a currently imprecise variable
instead of directly assigning 0 to them, like so:

static volatile u32 zero = 0;

for (i = zero; i < NUM_LOOPS; i++) {
	/* loop body */
}

The libarena implementation of this technique is currently faulty. For
the technique to work, the variable must not be in a map. This includes
the .rodata DATASEC map used for const variables. However, libarena
still defines the zero variable as constant.

Modify the zero variable definition into a volatile variable. This
change adds a complication caused by the compiler optimizing array
derefences from

for (i = zero; i < NUM_LOOPS; i++) {
	val = *(ptr + i);
}

into

for (i = zero; i < NUM_LOOPS; i++) {
	val = *ptr++;
}

and causing verification failures. Use the barrier_var() clobber macro
to prevent this optimization from taking place. Using barrier_var() is
the only way to break the optimization, as annotating the index as
volatile does not suffice.

After that, remove the bpf_for() invocations introduced in libarena for
parallel spmc testing.

Reported-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20260706181730.21731-3-emil@etsalapatis.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
This commit is contained in:
Emil Tsalapatis 2026-07-06 14:17:26 -04:00 • committed by Kumar Kartikeya Dwivedi
commit 857071efc3
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5 changed files with 18 additions and 12 deletions
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