On Fri, May 16, 2025 at 9:10 AM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
> Yi Lai reported an issue ([1]) where the following warning appears
> in kernel dmesg:
> [ 60.643604] verifier backtracking bug
> [ 60.643635] WARNING: CPU: 10 PID: 2315 at kernel/bpf/verifier.c:4302 __mark_chain_precision+0x3a6c/0x3e10
> [ 60.648428] Modules linked in: bpf_testmod(OE)
> [ 60.650471] CPU: 10 UID: 0 PID: 2315 Comm: test_progs Tainted: G OE 6.15.0-rc4-gef11287f8289-dirty #327 PREEMPT(full)
> [ 60.654385] Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
> [ 60.656682] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
> [ 60.660475] RIP: 0010:__mark_chain_precision+0x3a6c/0x3e10
> [ 60.662814] Code: 5a 30 84 89 ea e8 c4 d9 01 00 80 3d 3e 7d d8 04 00 0f 85 60 fa ff ff c6 05 31 7d d8 04
> 01 48 c7 c7 00 58 30 84 e8 c4 06 a5 ff <0f> 0b e9 46 fa ff ff 48 ...
> [ 60.668720] RSP: 0018:ffff888116cc7298 EFLAGS: 00010246
> [ 60.671075] RAX: 54d70e82dfd31900 RBX: ffff888115b65e20 RCX: 0000000000000000
> [ 60.673659] RDX: 0000000000000001 RSI: 0000000000000004 RDI: 00000000ffffffff
> [ 60.676241] RBP: 0000000000000400 R08: ffff8881f6f23bd3 R09: 1ffff1103ede477a
> [ 60.678787] R10: dffffc0000000000 R11: ffffed103ede477b R12: ffff888115b60ae8
> [ 60.681420] R13: 1ffff11022b6cbc4 R14: 00000000fffffff2 R15: 0000000000000001
> [ 60.684030] FS: 00007fc2aedd80c0(0000) GS:ffff88826fa8a000(0000) knlGS:0000000000000000
> [ 60.686837] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 60.689027] CR2: 000056325369e000 CR3: 000000011088b002 CR4: 0000000000370ef0
> [ 60.691623] Call Trace:
> [ 60.692821] <TASK>
> [ 60.693960] ? __pfx_verbose+0x10/0x10
> [ 60.695656] ? __pfx_disasm_kfunc_name+0x10/0x10
> [ 60.697495] check_cond_jmp_op+0x16f7/0x39b0
> [ 60.699237] do_check+0x58fa/0xab10
> ...
>
> Further analysis shows the warning is at line 4302 as below:
>
> 4294 /* static subprog call instruction, which
> 4295 * means that we are exiting current subprog,
> 4296 * so only r1-r5 could be still requested as
> 4297 * precise, r0 and r6-r10 or any stack slot in
> 4298 * the current frame should be zero by now
> 4299 */
> 4300 if (bt_reg_mask(bt) & ~BPF_REGMASK_ARGS) {
> 4301 verbose(env, "BUG regs %x\n", bt_reg_mask(bt));
> 4302 WARN_ONCE(1, "verifier backtracking bug");
> 4303 return -EFAULT;
> 4304 }
>
> With the below test (also in the next patch):
> __used __naked static void __bpf_jmp_r10(void)
> {
> asm volatile (
> "r2 = 2314885393468386424 ll;"
> "goto +0;"
> "if r2 <= r10 goto +3;"
> "if r1 >= -1835016 goto +0;"
> "if r2 <= 8 goto +0;"
> "if r3 <= 0 goto +0;"
> "exit;"
> ::: __clobber_all);
> }
>
> SEC("?raw_tp")
> __naked void bpf_jmp_r10(void)
> {
> asm volatile (
> "r3 = 0 ll;"
> "call __bpf_jmp_r10;"
> "r0 = 0;"
> "exit;"
> ::: __clobber_all);
> }
>
> The following is the verifier failure log:
> 0: (18) r3 = 0x0 ; R3_w=0
> 2: (85) call pc+2
> caller:
> R10=fp0
> callee:
> frame1: R1=ctx() R3_w=0 R10=fp0
> 5: frame1: R1=ctx() R3_w=0 R10=fp0
> ; asm volatile (" \ @ verifier_precision.c:184
> 5: (18) r2 = 0x20202000256c6c78 ; frame1: R2_w=0x20202000256c6c78
> 7: (05) goto pc+0
> 8: (bd) if r2 <= r10 goto pc+3 ; frame1: R2_w=0x20202000256c6c78 R10=fp0
> 9: (35) if r1 >= 0xffe3fff8 goto pc+0 ; frame1: R1=ctx()
> 10: (b5) if r2 <= 0x8 goto pc+0
> mark_precise: frame1: last_idx 10 first_idx 0 subseq_idx -1
> mark_precise: frame1: regs=r2 stack= before 9: (35) if r1 >= 0xffe3fff8 goto pc+0
> mark_precise: frame1: regs=r2 stack= before 8: (bd) if r2 <= r10 goto pc+3
> mark_precise: frame1: regs=r2,r10 stack= before 7: (05) goto pc+0
> mark_precise: frame1: regs=r2,r10 stack= before 5: (18) r2 = 0x20202000256c6c78
> mark_precise: frame1: regs=r10 stack= before 2: (85) call pc+2
> BUG regs 400
>
> The main failure reason is due to r10 in precision backtracking bookkeeping.
> Actually r10 is always precise and there is no need to add it the precision
> backtracking bookkeeping.
>
> One way to fix the issue is to prevent bt_set_reg() if any src/dst reg is
> r10. Andrii suggested to go with push_insn_history() approach to avoid
> explicitly checking r10 in backtrack_insn().
>
> This patch added push_insn_history() support for cond_jmp like 'rX <op> rY'
> operations. In check_cond_jmp_op(), if any of rX or rY is r10, push_insn_history()
> will not record that register, and later backtrack_insn() will not do
> bt_set_reg() for those registers.
>
> [1] https://lore.kernel.org/bpf/Z%2F8q3xzpU59CIYQE@ly-workstation/
>
> Reported by: Yi Lai <yi1.lai@xxxxxxxxxxxxxxx>
> Fixes: 407958a0e980 ("bpf: encapsulate precision backtracking bookkeeping")
> Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
> ---
> include/linux/bpf_verifier.h | 7 ++++
> kernel/bpf/verifier.c | 69 +++++++++++++++++++++++++++++-------
> 2 files changed, 64 insertions(+), 12 deletions(-)
>
> Changelogs:
> v1 -> v2:
> - v1: https://lore.kernel.org/bpf/20250511162758.281071-1-yonghong.song@xxxxxxxxx/
> - In v1, we check r10 register explicitly in backtrack_insn() to decide
> whether we should do bt_set_reg() or not. Andrii suggested to do
> push_insn_history() instead. Whether a particular register (r10 in this case)
> should be available for backtracking or not is in check_cond_jmp_op(),
> and such information is pushed with push_insn_history(). Later in backtrack_insn(),
> such info is retrieved to decide whether precision marking should be
> done or not. This apporach can avoid explicit checking for r10 in backtrack_insn().
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index cedd66867ecf..9d3fdabeeaf4 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -357,6 +357,8 @@ enum {
> INSN_F_SPI_SHIFT = 3, /* shifted 3 bits to the left */
>
> INSN_F_STACK_ACCESS = BIT(9), /* we need 10 bits total */
> +
> + INSN_F_REG_ACCESS = BIT(4), /* we need 5 bits total */
hm... you are actually clashing with INSN_F_SPI_MASK here, bits 3
through 8 are used to record stack slot index.
I think we should go with
INSN_F_DST_REG_STACK = BIT(10), /* dst_reg is PTR_TO_STACK */
INSN_F_SRC_REG_STACK = BIT(11), /* src_reg is PTR_TO_STACK */
/* total 12 bits total can used now */
also note that all this stuff needs to fit into
bpf_insn_hist_entry.flags, which is currently set to be 10 bits, and
so now we need two extra bits.
Luckily, prev_idx: 22 doesn't really need 22 bits, so we can steal two
bits there and still be able to express 1 million instructions
indices:
u32 prev_idx : 20;
u32 flags : 12;
pw-bot: cr
> };
>
> static_assert(INSN_F_FRAMENO_MASK + 1 >= MAX_CALL_FRAMES);
> @@ -372,6 +374,11 @@ struct bpf_insn_hist_entry {
> * jump is backtracked, vector of six 10-bit records
> */
> u64 linked_regs;
> + /* special flag, e.g., whether reg is used for non-load/store insns
> + * during precision backtracking.
> + */
> + u8 sreg_flag;
> + u8 dreg_flag;
this is not necessary, src_reg and dst_reg number itself is coming
from the bpf_insn itself?
[...]
> @@ -4414,8 +4425,16 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
> * before it would be equally necessary to
> * propagate it to dreg.
> */
> - bt_set_reg(bt, dreg);
> - bt_set_reg(bt, sreg);
> + if (!hist)
> + return 0;
> + dreg_precise = hist->dreg_flag == insn_reg_with_access_flag(dreg);
> + sreg_precise = hist->sreg_flag == insn_reg_with_access_flag(sreg);
As I mentioned above, we don't need to store dst_reg and src_reg
numbers themselves, we are getting them from bpf_insn. We just need
those two flags, INSN_F_DST_REG_STACK and INSN_F_SRC_REG_STACK, to
know which registers were PTR_TO_STACK at the time when we validated
that instruction
> + if (!dreg_precise && !sreg_precise)
> + return 0;
> + if (dreg_precise)
> + bt_set_reg(bt, dreg);
> + if (sreg_precise)
> + bt_set_reg(bt, sreg);
> } else if (BPF_SRC(insn->code) == BPF_K) {
> /* dreg <cond> K
> * Only dreg still needs precision before
> @@ -5115,7 +5134,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> }
>
> if (insn_flags)
> - return push_insn_history(env, env->cur_state, insn_flags, 0);
> + return push_insn_history(env, env->cur_state, insn_flags, 0, 0, 0);
> return 0;
> }
>
> @@ -5422,7 +5441,7 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
> insn_flags = 0; /* we are not restoring spilled register */
> }
> if (insn_flags)
> - return push_insn_history(env, env->cur_state, insn_flags, 0);
> + return push_insn_history(env, env->cur_state, insn_flags, 0, 0, 0);
> return 0;
> }
>
> @@ -16414,6 +16433,27 @@ static void sync_linked_regs(struct bpf_verifier_state *vstate, struct bpf_reg_s
> }
> }
>
> +static int push_cond_jmp_history(struct bpf_verifier_env *env, struct bpf_verifier_state *state,
> + struct bpf_insn *insn, u64 linked_regs)
> +{
> + int err;
> +
> + if ((BPF_SRC(insn->code) != BPF_X ||
> + (insn->src_reg == BPF_REG_FP && insn->dst_reg == BPF_REG_FP)) &&
we shouldn't be checking for BPF_REG_FP here either. Look up
bpf_reg_state by insn->src_reg, and check that it's PTR_TO_STACK
> + !linked_regs)
> + return 0;
> +
> + err = push_insn_history(env, state, 0, linked_regs,
> + BPF_SRC(insn->code) == BPF_X && insn->src_reg != BPF_REG_FP
> + ? insn_reg_with_access_flag(insn->src_reg)
> + : 0,
> + BPF_SRC(insn->code) == BPF_X && insn->dst_reg != BPF_REG_FP
> + ? insn_reg_with_access_flag(insn->dst_reg)
> + : 0);
> +
> + return err;
> +}
> +
[...]
