Merge branch 'a-tool-to-verify-the-bpf-memory-model'

Puranjay Mohan says:

====================
A tool to verify the BPF memory model

I am building a tool called blitmus[1] that converts memory model litmus
tests written in C into BPF programs that run in parallel to verify that the
JITs are enforcing the memory model correctly.

With this tool I was able to find a bug in the implementation of the smp_mb()
in the selftests.

Using the following litmus test:

C SB+fencembonceonces

(*
 * Result: Never
 *
 * This litmus test demonstrates that full memory barriers suffice to
 * order the store-buffering pattern, where each process writes to the
 * variable that the preceding process reads.  (Locking and RCU can also
 * suffice, but not much else.)
 *)

{}

P0(int *x, int *y)
{
        int r0;

        WRITE_ONCE(*x, 1);
        smp_mb();
        r0 = READ_ONCE(*y);
}

P1(int *x, int *y)
{
        int r0;

        WRITE_ONCE(*y, 1);
        smp_mb();
        r0 = READ_ONCE(*x);
}

exists (0:r0=0 /\ 1:r0=0)

Running the generated program on an ARMv8 machine:

With the current implementation of smp_mb():

    [root@fedora blitmus]# ./sb_fencembonceonces
    Starting litmus test with configuration:
      Test: SB+fencembonceonces
      Iterations: 4100

    Test SB+fencembonceonces Allowed
    Histogram (4 states)
    4545     *>0:r0=0; 1:r0=0;
    20403742 :>0:r0=0; 1:r0=1;
    20591700 :>0:r0=1; 1:r0=0;
    13       :>0:r0=1; 1:r0=1;
    Ok

    Witnesses
    Positive: 4545, Negative: 40995455
    Condition exists (0:r0=0 /\ 1:r0=0) is validated
    Observation SB+fencembonceonces Sometimes 4545 40995455
    Time SB+fencembonceonces 8.33

    Thu Jul 10 16:56:41 UTC

Positive witnesses mean that smp_mb() is not working as
expected and not providing any ordering.

After applying the patch to fix smp_mb():

    [root@fedora blitmus]# ./sb_fencembonceonces
    Starting litmus test with configuration:
      Test: SB+fencembonceonces
      Iterations: 4100

    Test SB+fencembonceonces Allowed
    Histogram (3 states)
    19657569 :>0:r0=0; 1:r0=1;
    20227574 :>0:r0=1; 1:r0=0;
    1114857  :>0:r0=1; 1:r0=1;
    No

    Witnesses
    Positive: 0, Negative: 41000000
    Condition exists (0:r0=0 /\ 1:r0=0) is NOT validated
    Observation SB+fencembonceonces Never 0 41000000
    Time SB+fencembonceonces 9.58

    Thu Jul 10 16:56:10 UTC

0 positive witnesses mean that invalid behaviour is not seen and smp_mb()
is ordering the operations properly.

I hope to improve this tool more and use it to fuzz the JITs of ARMv8,
RISC-V, and Power and see what other bugs can be exposed.

[1] https://github.com/puranjaymohan/blitmus
====================

Link: https://patch.msgid.link/20250710175434.18829-1-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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