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Date: Tue, 22 Nov 2022 13:03:55 +0530
From: "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>,
Michael Ellerman <mpe@...erman.id.au>,
Nicholas Piggin <npiggin@...il.com>
Cc: Andrii Nakryiko <andrii@...nel.org>,
Alexei Starovoitov <ast@...nel.org>, bpf@...r.kernel.org,
Daniel Borkmann <daniel@...earbox.net>,
Hao Luo <haoluo@...gle.com>,
John Fastabend <john.fastabend@...il.com>,
Jiri Olsa <jolsa@...nel.org>, KP Singh <kpsingh@...nel.org>,
linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
Martin KaFai Lau <martin.lau@...ux.dev>,
Stanislav Fomichev <sdf@...gle.com>,
Song Liu <song@...nel.org>, stable@...r.kernel.org,
Yonghong Song <yhs@...com>
Subject: Re: [PATCH] powerpc/bpf/32: Fix Oops on tail call tests
Christophe Leroy wrote:
> test_bpf tail call tests end up as:
>
> test_bpf: #0 Tail call leaf jited:1 85 PASS
> test_bpf: #1 Tail call 2 jited:1 111 PASS
> test_bpf: #2 Tail call 3 jited:1 145 PASS
> test_bpf: #3 Tail call 4 jited:1 170 PASS
> test_bpf: #4 Tail call load/store leaf jited:1 190 PASS
> test_bpf: #5 Tail call load/store jited:1
> BUG: Unable to handle kernel data access on write at 0xf1b4e000
> Faulting instruction address: 0xbe86b710
> Oops: Kernel access of bad area, sig: 11 [#1]
> BE PAGE_SIZE=4K MMU=Hash PowerMac
> Modules linked in: test_bpf(+)
> CPU: 0 PID: 97 Comm: insmod Not tainted 6.1.0-rc4+ #195
> Hardware name: PowerMac3,1 750CL 0x87210 PowerMac
> NIP: be86b710 LR: be857e88 CTR: be86b704
> REGS: f1b4df20 TRAP: 0300 Not tainted (6.1.0-rc4+)
> MSR: 00009032 <EE,ME,IR,DR,RI> CR: 28008242 XER: 00000000
> DAR: f1b4e000 DSISR: 42000000
> GPR00: 00000001 f1b4dfe0 c11d2280 00000000 00000000 00000000 00000002 00000000
> GPR08: f1b4e000 be86b704 f1b4e000 00000000 00000000 100d816a f2440000 fe73baa8
> GPR16: f2458000 00000000 c1941ae4 f1fe2248 00000045 c0de0000 f2458030 00000000
> GPR24: 000003e8 0000000f f2458000 f1b4dc90 3e584b46 00000000 f24466a0 c1941a00
> NIP [be86b710] 0xbe86b710
> LR [be857e88] __run_one+0xec/0x264 [test_bpf]
> Call Trace:
> [f1b4dfe0] [00000002] 0x2 (unreliable)
> Instruction dump:
> XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
> XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
> ---[ end trace 0000000000000000 ]---
>
> This is a tentative to write above the stack. The problem is encoutered
> with tests added by commit 38608ee7b690 ("bpf, tests: Add load store
> test case for tail call")
>
> This happens because tail call is done to a BPF prog with a different
> stack_depth. At the time being, the stack is kept as is when the caller
> tail calls its callee. But at exit, the callee restores the stack based
> on its own properties. Therefore here, at each run, r1 is erroneously
> increased by 32 - 16 = 16 bytes.
>
> This was done that way in order to pass the tail call count from caller
> to callee through the stack. As powerpc32 doesn't have a red zone in
> the stack, it was necessary the maintain the stack as is for the tail
> call. But it was not anticipated that the BPF frame size could be
> different.
>
> Let's take a new approach. Use register r0 to carry the tail call count
> during the tail call, and save it into the stack at function entry if
> required. That's a deviation from the ppc32 ABI, but after all the way
> tail calls are implemented is already not in accordance with the ABI.
Can we pass the tail call count in r4 instead?
>
> With the fix, tail call tests are now successfull:
>
> test_bpf: #0 Tail call leaf jited:1 53 PASS
> test_bpf: #1 Tail call 2 jited:1 115 PASS
> test_bpf: #2 Tail call 3 jited:1 154 PASS
> test_bpf: #3 Tail call 4 jited:1 165 PASS
> test_bpf: #4 Tail call load/store leaf jited:1 101 PASS
> test_bpf: #5 Tail call load/store jited:1 141 PASS
> test_bpf: #6 Tail call error path, max count reached jited:1 994 PASS
> test_bpf: #7 Tail call count preserved across function calls jited:1 140975 PASS
> test_bpf: #8 Tail call error path, NULL target jited:1 110 PASS
> test_bpf: #9 Tail call error path, index out of range jited:1 69 PASS
> test_bpf: test_tail_calls: Summary: 10 PASSED, 0 FAILED, [10/10 JIT'ed]
>
> Fixes: 51c66ad849a7 ("powerpc/bpf: Implement extended BPF on PPC32")
> Cc: stable@...r.kernel.org
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
> arch/powerpc/net/bpf_jit_comp32.c | 25 +++++++++++--------------
> 1 file changed, 11 insertions(+), 14 deletions(-)
>
> diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
> index 43f1c76d48ce..97e75b8181ca 100644
> --- a/arch/powerpc/net/bpf_jit_comp32.c
> +++ b/arch/powerpc/net/bpf_jit_comp32.c
> @@ -115,21 +115,19 @@ void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
>
> /* First arg comes in as a 32 bits pointer. */
> EMIT(PPC_RAW_MR(bpf_to_ppc(BPF_REG_1), _R3));
> - EMIT(PPC_RAW_LI(bpf_to_ppc(BPF_REG_1) - 1, 0));
> + EMIT(PPC_RAW_LI(_R0, 0));
> +
> +#define BPF_TAILCALL_PROLOGUE_SIZE 8
> +
> EMIT(PPC_RAW_STWU(_R1, _R1, -BPF_PPC_STACKFRAME(ctx)));
>
> /*
> - * Initialize tail_call_cnt in stack frame if we do tail calls.
> - * Otherwise, put in NOPs so that it can be skipped when we are
> - * invoked through a tail call.
> + * Save tail_call_cnt in stack frame if we do tail calls.
> */
> if (ctx->seen & SEEN_TAILCALL)
> - EMIT(PPC_RAW_STW(bpf_to_ppc(BPF_REG_1) - 1, _R1,
> - bpf_jit_stack_offsetof(ctx, BPF_PPC_TC)));
> - else
> - EMIT(PPC_RAW_NOP());
> + EMIT(PPC_RAW_STW(_R0, _R1, bpf_jit_stack_offsetof(ctx, BPF_PPC_TC)));
>
> -#define BPF_TAILCALL_PROLOGUE_SIZE 16
> + EMIT(PPC_RAW_LI(bpf_to_ppc(BPF_REG_1) - 1, 0));
>
> /*
> * We need a stack frame, but we don't necessarily need to
> @@ -244,7 +242,6 @@ static int bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32 o
> EMIT(PPC_RAW_RLWINM(_R3, b2p_index, 2, 0, 29));
> EMIT(PPC_RAW_ADD(_R3, _R3, b2p_bpf_array));
> EMIT(PPC_RAW_LWZ(_R3, _R3, offsetof(struct bpf_array, ptrs)));
> - EMIT(PPC_RAW_STW(_R0, _R1, bpf_jit_stack_offsetof(ctx, BPF_PPC_TC)));
>
> /*
> * if (prog == NULL)
> @@ -257,20 +254,20 @@ static int bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32 o
> EMIT(PPC_RAW_LWZ(_R3, _R3, offsetof(struct bpf_prog, bpf_func)));
>
> if (ctx->seen & SEEN_FUNC)
> - EMIT(PPC_RAW_LWZ(_R0, _R1, BPF_PPC_STACKFRAME(ctx) + PPC_LR_STKOFF));
> + EMIT(PPC_RAW_LWZ(_R5, _R1, BPF_PPC_STACKFRAME(ctx) + PPC_LR_STKOFF));
>
> EMIT(PPC_RAW_ADDIC(_R3, _R3, BPF_TAILCALL_PROLOGUE_SIZE));
>
> if (ctx->seen & SEEN_FUNC)
> - EMIT(PPC_RAW_MTLR(_R0));
> + EMIT(PPC_RAW_MTLR(_R5));
Should we explicitly zero-out _R5 after this?
You can move the above PPC_RAW_LWZ() and PPC_RAW_MTLR() instructions, as
well as the ADDI below for r1 into bpf_jit_emit_common_epilogue() and
not have to repeat those here.
- Naveen
>
> EMIT(PPC_RAW_MTCTR(_R3));
>
> - EMIT(PPC_RAW_MR(_R3, bpf_to_ppc(BPF_REG_1)));
> -
> /* tear restore NVRs, ... */
> bpf_jit_emit_common_epilogue(image, ctx);
>
> + EMIT(PPC_RAW_ADDI(_R1, _R1, BPF_PPC_STACKFRAME(ctx)));
> +
> EMIT(PPC_RAW_BCTR());
>
> /* out: */
> --
> 2.38.1
>
>
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