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Sun, 06 Sep 2026 22:42:50 -0700 (PDT) From: Donggeun Yoo To: Alexei Starovoitov , Andrii Nakryiko , Catalin Marinas , Daniel Borkmann , Eduard Zingerman , Emil Tsalapatis , Ihor Solodrai , Jiri Olsa , Kumar Kartikeya Dwivedi , Mark Rutland , Martin KaFai Lau , Puranjay Mohan , Shuah Khan , Song Liu , Will Deacon , Xu Kuohai , Yonghong Song Cc: bpf@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, donggeunyoo.kernel@gmail.com Subject: [PATCH bpf v2 1/2] bpf, arm64: set up the frame pointer for the exception callback Date: Mon, 7 Sep 2026 14:42:34 +0900 Message-ID: <20260907054235.473103-2-donggeunyoo.kernel@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260907054235.473103-1-donggeunyoo.kernel@gmail.com> References: <20260907054235.473103-1-donggeunyoo.kernel@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit A program acting as exception boundary saves all callee-saved registers, so build_prologue() takes the exception_cb path and never calls push_callee_regs(). That is the only place find_used_callee_regs() runs, and with it the only place ctx->fp_used is set, so the callback prologue does not emit the mov x25, sp that points BPF_REG_FP at the frame the callback runs on. x25 keeps whatever it held when bpf_throw() was called. If the throw came from a subprogram that uses its own BPF stack, that is the subprogram's frame pointer, and since the subprogram never returns it never restores x25 either. Stack accesses through BPF_REG_FP are rewritten to be stack pointer relative, so those still land in the callback's own frame. Materializing the register does not: a callback that passes the address of a local variable to a helper hands over an address in the dead subprogram's frame. That address is below the callback's stack pointer by then, and the helper's own call chain covers it, so the helper can write over its own return address. 0x1234 below is the value the helper was asked to store: pc : 0x1234 lr : 0x1234 Call trace: 0x1234 (P) bpf_test_run+0x188/0x3e0 bpf_prog_test_run_skb+0x47c/0x998 __sys_bpf+0xbdc/0xdd8 Kernel panic - not syncing: Oops: Fatal exception in interrupt Set ctx->fp_used on the exception callback path so that the existing code further down sets x25 from the stack pointer. The epilogue restores it from the main program's save area along with the other callee-saved registers, as it already does. x86 sets the frame pointer for the callback from the argument it is passed, and powerpc computes it from the stack pointer. Fixes: 5d4fa9ec5643 ("bpf, arm64: Avoid blindly saving/restoring all callee-saved registers") Signed-off-by: Donggeun Yoo --- arch/arm64/net/bpf_jit_comp.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c index c18e005a41db..c5f55d6161fe 100644 --- a/arch/arm64/net/bpf_jit_comp.c +++ b/arch/arm64/net/bpf_jit_comp.c @@ -600,6 +600,8 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf) * 12 registers are on the stack */ emit(A64_SUB_I(1, A64_SP, A64_FP, 96), ctx); + /* The callback may use its own BPF stack, set up fp for it. */ + ctx->fp_used = true; } /* Stack must be multiples of 16B */ -- 2.53.0