[PATCH bpf-next v1 1/7] bpf: Fix timed may_goto stack pointer for private stacks

[Siddharth Chintamaneni <sidchintamaneni@gmail.com>]

timed may_goto passes a stack offset to the architecture trampoline,
which reconstructs the counter pointer from its BPF frame pointer. This
breaks when the JIT uses a private stack with a different frame pointer.

Resolve the counter pointer in the fixup using BPF_REG_FP and pass the
pointer through BPF_REG_AX. Account for the extra instruction in the
internal branch offsets.

Fixes: e723608bf428 ("bpf: Add verifier support for timed may_goto")
Reported-by: Jeremy Jean <jeremy.jean@oss.cyber.gouv.fr>
Link: https://lore.kernel.org/all/20260824213158.3755932-2-Jeremy.Jean@oss.cyber.gouv.fr/
Signed-off-by: Siddharth Chintamaneni <sidchintamaneni@gmail.com>
---
 kernel/bpf/fixups.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/kernel/bpf/fixups.c b/kernel/bpf/fixups.c
index 65b441e4a351..dc59501a32bb 100644
--- a/kernel/bpf/fixups.c
+++ b/kernel/bpf/fixups.c
@@ -1797,20 +1797,20 @@ int bpf_do_misc_fixups(struct bpf_verifier_env *env)
 			stack_depth_extra = 16;
 			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_AX, BPF_REG_10, stack_off_cnt);
 			if (insn->off >= 0)
-				insn_buf[1] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, insn->off + 5);
+				insn_buf[1] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, insn->off + 6);
 			else
 				insn_buf[1] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, insn->off - 1);
 			insn_buf[2] = BPF_ALU64_IMM(BPF_SUB, BPF_REG_AX, 1);
-			insn_buf[3] = BPF_JMP_IMM(BPF_JNE, BPF_REG_AX, 0, 2);
+			insn_buf[3] = BPF_JMP_IMM(BPF_JNE, BPF_REG_AX, 0, 3);
 			/*
-			 * AX is used as an argument to pass in stack_off_cnt
-			 * (to add to r10/fp), and also as the return value of
-			 * the call to arch_bpf_timed_may_goto.
+			 * AX is used to pass FP + stack_off_cnt as the argument to
+			 * arch_bpf_timed_may_goto(), and also holds its return value.
 			 */
-			insn_buf[4] = BPF_MOV64_IMM(BPF_REG_AX, stack_off_cnt);
-			insn_buf[5] = BPF_EMIT_CALL(arch_bpf_timed_may_goto);
-			insn_buf[6] = BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_AX, stack_off_cnt);
-			cnt = 7;
+			insn_buf[4] = BPF_MOV64_REG(BPF_REG_AX, BPF_REG_FP);
+			insn_buf[5] = BPF_ALU64_IMM(BPF_ADD, BPF_REG_AX, stack_off_cnt);
+			insn_buf[6] = BPF_EMIT_CALL(arch_bpf_timed_may_goto);
+			insn_buf[7] = BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_AX, stack_off_cnt);
+			cnt = 8;
 
 			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
 			if (!new_prog)
@@ -2661,4 +2661,3 @@ int bpf_remove_fastcall_spills_fills(struct bpf_verifier_env *env)
 
 	return 0;
 }
-
-- 
2.43.0