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selftests/bpf: test cases for bpf_loop SCC and state graph backedges

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Test for state graph backedges accumulation for SCCs formed by
bpf_loop(). Equivalent to the following C program:

  int main(void) {
    1: fp[-8] = bpf_get_prandom_u32();
    2: fp[-16] = -32;                       // used in a memory access below
    3: bpf_loop(7, loop_cb4, fp, 0);
    4: return 0;
  }

  int loop_cb4(int i, void *ctx) {
    5: if (unlikely(ctx[-8] > bpf_get_prandom_u32()))
    6:   *(u64 *)(fp + ctx[-16]) = 42;      // aligned access expected
    7: if (unlikely(fp[-8] > bpf_get_prandom_u32()))
    8:   ctx[-16] = -31;                    // makes said access unaligned
    9: return 0;
  }

If state graph backedges are not accumulated properly at the SCC
formed by loop_cb4() call from bpf_loop(), the state {ctx[-16]=-32}
injected at instruction 9 on verification path 1,2,3,5,7,9,4 would be
considered fully verified and would lack precision mark for ctx[-16].
This would lead to early pruning of verification path 1,2,3,5,7,8,9 in
state {ctx[-16]=-31}, which in turn leads to the incorrect assumption
that the above program is safe.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251229-scc-for-callbacks-v1-2-ceadfe679900@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Eduard Zingerman 2025-12-29 23:13:08 -08:00 committed by Alexei Starovoitov
parent f597664454
commit e6f2612f0e
1 changed files with 75 additions and 0 deletions
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75
tools/testing/selftests/bpf/progs/iters.c
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@ -1926,4 +1926,79 @@ static int loop1_wrapper(void)
);
}
/*
* This is similar to a test case absent_mark_in_the_middle_state(),
* but adapted for use with bpf_loop().
*/
SEC("raw_tp")
__flag(BPF_F_TEST_STATE_FREQ)
__failure __msg("math between fp pointer and register with unbounded min value is not allowed")
__naked void absent_mark_in_the_middle_state4(void)
{
/*
* Equivalent to a C program below:
*
* int main(void) {
* fp[-8] = bpf_get_prandom_u32();
* fp[-16] = -32; // used in a memory access below
* bpf_loop(7, loop_cb4, fp, 0);
* return 0;
* }
*
* int loop_cb4(int i, void *ctx) {
* if (unlikely(ctx[-8] > bpf_get_prandom_u32()))
* *(u64 *)(fp + ctx[-16]) = 42; // aligned access expected
* if (unlikely(fp[-8] > bpf_get_prandom_u32()))
* ctx[-16] = -31; // makes said access unaligned
* return 0;
* }
*/
asm volatile (
"call %[bpf_get_prandom_u32];"
"r8 = r0;"
"*(u64 *)(r10 - 8) = r0;"
"*(u64 *)(r10 - 16) = -32;"
"r1 = 7;"
"r2 = loop_cb4 ll;"
"r3 = r10;"
"r4 = 0;"
"call %[bpf_loop];"
"r0 = 0;"
"exit;"
:
: __imm(bpf_loop),
__imm(bpf_get_prandom_u32)
: __clobber_all
);
}
__used __naked
static void loop_cb4(void)
{
asm volatile (
"r9 = r2;"
"r8 = *(u64 *)(r9 - 8);"
"r6 = *(u64 *)(r9 - 16);"
"call %[bpf_get_prandom_u32];"
"if r0 > r8 goto use_fp16_%=;"
"1:"
"call %[bpf_get_prandom_u32];"
"if r0 > r8 goto update_fp16_%=;"
"2:"
"r0 = 0;"
"exit;"
"use_fp16_%=:"
"r1 = r10;"
"r1 += r6;"
"*(u64 *)(r1 + 0) = 42;"
"goto 1b;"
"update_fp16_%=:"
"*(u64 *)(r9 - 16) = -31;"
"goto 2b;"
:
: __imm(bpf_get_prandom_u32)
: __clobber_all
);
}
char _license[] SEC("license") = "GPL";