[PATCH bpf-next 1/2] bpf: Remove the insn_buf array stack usage from the inline_bpf_loop()

[Martin KaFai Lau <martin.lau@linux.dev>]

From: Martin KaFai Lau <martin.lau@kernel.org>

This patch removes the insn_buf array stack usage from the
inline_bpf_loop(). Instead, the env->insn_buf is used. The
usage in inline_bpf_loop() needs more than 16 insn, so the
INSN_BUF_SIZE needs to be increased from 16 to 32.
The compiler stack size warning on the verifier is gone
after this change.

Cc: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
---
 include/linux/bpf_verifier.h |  2 +-
 kernel/bpf/verifier.c        | 83 ++++++++++++++++++------------------
 2 files changed, 43 insertions(+), 42 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 2e20207315a9..8458632824a4 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -24,7 +24,7 @@
  */
 #define TMP_STR_BUF_LEN 320
 /* Patch buffer size */
-#define INSN_BUF_SIZE 16
+#define INSN_BUF_SIZE 32
 
 /* Liveness marks, used for registers and spilled-regs (in stack slots).
  * Read marks propagate upwards until they find a write mark; they record that
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 217eb0eafa2a..8bfb14d1eb7a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -21232,7 +21232,7 @@ static struct bpf_prog *inline_bpf_loop(struct bpf_verifier_env *env,
 					int position,
 					s32 stack_base,
 					u32 callback_subprogno,
-					u32 *cnt)
+					u32 *total_cnt)
 {
 	s32 r6_offset = stack_base + 0 * BPF_REG_SIZE;
 	s32 r7_offset = stack_base + 1 * BPF_REG_SIZE;
@@ -21241,55 +21241,56 @@ static struct bpf_prog *inline_bpf_loop(struct bpf_verifier_env *env,
 	int reg_loop_cnt = BPF_REG_7;
 	int reg_loop_ctx = BPF_REG_8;
 
+	struct bpf_insn *insn_buf = env->insn_buf;
 	struct bpf_prog *new_prog;
 	u32 callback_start;
 	u32 call_insn_offset;
 	s32 callback_offset;
+	u32 cnt = 0;
 
 	/* This represents an inlined version of bpf_iter.c:bpf_loop,
 	 * be careful to modify this code in sync.
 	 */
-	struct bpf_insn insn_buf[] = {
-		/* Return error and jump to the end of the patch if
-		 * expected number of iterations is too big.
-		 */
-		BPF_JMP_IMM(BPF_JLE, BPF_REG_1, BPF_MAX_LOOPS, 2),
-		BPF_MOV32_IMM(BPF_REG_0, -E2BIG),
-		BPF_JMP_IMM(BPF_JA, 0, 0, 16),
-		/* spill R6, R7, R8 to use these as loop vars */
-		BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_6, r6_offset),
-		BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_7, r7_offset),
-		BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_8, r8_offset),
-		/* initialize loop vars */
-		BPF_MOV64_REG(reg_loop_max, BPF_REG_1),
-		BPF_MOV32_IMM(reg_loop_cnt, 0),
-		BPF_MOV64_REG(reg_loop_ctx, BPF_REG_3),
-		/* loop header,
-		 * if reg_loop_cnt >= reg_loop_max skip the loop body
-		 */
-		BPF_JMP_REG(BPF_JGE, reg_loop_cnt, reg_loop_max, 5),
-		/* callback call,
-		 * correct callback offset would be set after patching
-		 */
-		BPF_MOV64_REG(BPF_REG_1, reg_loop_cnt),
-		BPF_MOV64_REG(BPF_REG_2, reg_loop_ctx),
-		BPF_CALL_REL(0),
-		/* increment loop counter */
-		BPF_ALU64_IMM(BPF_ADD, reg_loop_cnt, 1),
-		/* jump to loop header if callback returned 0 */
-		BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, -6),
-		/* return value of bpf_loop,
-		 * set R0 to the number of iterations
-		 */
-		BPF_MOV64_REG(BPF_REG_0, reg_loop_cnt),
-		/* restore original values of R6, R7, R8 */
-		BPF_LDX_MEM(BPF_DW, BPF_REG_6, BPF_REG_10, r6_offset),
-		BPF_LDX_MEM(BPF_DW, BPF_REG_7, BPF_REG_10, r7_offset),
-		BPF_LDX_MEM(BPF_DW, BPF_REG_8, BPF_REG_10, r8_offset),
-	};
 
-	*cnt = ARRAY_SIZE(insn_buf);
-	new_prog = bpf_patch_insn_data(env, position, insn_buf, *cnt);
+	/* Return error and jump to the end of the patch if
+	 * expected number of iterations is too big.
+	 */
+	insn_buf[cnt++] = BPF_JMP_IMM(BPF_JLE, BPF_REG_1, BPF_MAX_LOOPS, 2);
+	insn_buf[cnt++] = BPF_MOV32_IMM(BPF_REG_0, -E2BIG);
+	insn_buf[cnt++] = BPF_JMP_IMM(BPF_JA, 0, 0, 16);
+	/* spill R6, R7, R8 to use these as loop vars */
+	insn_buf[cnt++] = BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_6, r6_offset);
+	insn_buf[cnt++] = BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_7, r7_offset);
+	insn_buf[cnt++] = BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_8, r8_offset);
+	/* initialize loop vars */
+	insn_buf[cnt++] = BPF_MOV64_REG(reg_loop_max, BPF_REG_1);
+	insn_buf[cnt++] = BPF_MOV32_IMM(reg_loop_cnt, 0);
+	insn_buf[cnt++] = BPF_MOV64_REG(reg_loop_ctx, BPF_REG_3);
+	/* loop header,
+	 * if reg_loop_cnt >= reg_loop_max skip the loop body
+	 */
+	insn_buf[cnt++] = BPF_JMP_REG(BPF_JGE, reg_loop_cnt, reg_loop_max, 5);
+	/* callback call,
+	 * correct callback offset would be set after patching
+	 */
+	insn_buf[cnt++] = BPF_MOV64_REG(BPF_REG_1, reg_loop_cnt);
+	insn_buf[cnt++] = BPF_MOV64_REG(BPF_REG_2, reg_loop_ctx);
+	insn_buf[cnt++] = BPF_CALL_REL(0);
+	/* increment loop counter */
+	insn_buf[cnt++] = BPF_ALU64_IMM(BPF_ADD, reg_loop_cnt, 1);
+	/* jump to loop header if callback returned 0 */
+	insn_buf[cnt++] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, -6);
+	/* return value of bpf_loop,
+	 * set R0 to the number of iterations
+	 */
+	insn_buf[cnt++] = BPF_MOV64_REG(BPF_REG_0, reg_loop_cnt);
+	/* restore original values of R6, R7, R8 */
+	insn_buf[cnt++] = BPF_LDX_MEM(BPF_DW, BPF_REG_6, BPF_REG_10, r6_offset);
+	insn_buf[cnt++] = BPF_LDX_MEM(BPF_DW, BPF_REG_7, BPF_REG_10, r7_offset);
+	insn_buf[cnt++] = BPF_LDX_MEM(BPF_DW, BPF_REG_8, BPF_REG_10, r8_offset);
+
+	*total_cnt = cnt;
+	new_prog = bpf_patch_insn_data(env, position, insn_buf, cnt);
 	if (!new_prog)
 		return new_prog;
 
-- 
2.43.5