Commit e6f2612f authored by Eduard Zingerman's avatar Eduard Zingerman Committed by Alexei Starovoitov
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selftests/bpf: test cases for bpf_loop SCC and state graph backedges 



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: default avatarEduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251229-scc-for-callbacks-v1-2-ceadfe679900@gmail.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent f5976644

tools/testing/selftests/bpf/progs/iters.c

+75 −0
Original line number Diff line number Diff line
@@ -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";