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Mon, 14 Sep 2026 19:13:57 -0700 (PDT) From: Nicholas Dudar To: ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, eddyz87@gmail.com, memxor@gmail.com, udknight@gmail.com, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org Cc: martin.lau@linux.dev, song@kernel.org, yonghong.song@linux.dev, jolsa@kernel.org, emil@etsalapatis.com, ihor.solodrai@linux.dev, hpa@zytor.com, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, visitorckw@gmail.com Subject: [PATCH bpf-next 1/5] bpf, x86: Add support for BPF_MOVSX in the i386 JIT Date: Mon, 14 Sep 2026 22:13:34 -0400 Message-ID: <20260915021338.1520964-2-main.kalliope@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260915021338.1520964-1-main.kalliope@gmail.com> References: <20260915021338.1520964-1-main.kalliope@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 The i386 JIT lowers register BPF_MOVSX as an ordinary move because do_jit() does not inspect insn->off. The generated code therefore copies the source instead of sign-extending the selected low bits. Context-access conversion can also introduce MOVSX while lowering signed loads, so rejecting BPF_MEMSX does not protect those paths. Stage the source low word in EAX and use the native byte and word sign-extension instructions. Derive the high word from the sign bit for ALU64 and retain verifier-managed zero extension for ALU32. Assisted-by: Codex:gpt-6 Signed-off-by: Nicholas Dudar --- arch/x86/net/bpf_jit_comp32.c | 51 +++++++++++++++++++++++++++++++++++ 1 file changed, 51 insertions(+) diff --git a/arch/x86/net/bpf_jit_comp32.c b/arch/x86/net/bpf_jit_comp32.c index 852baf2e4db4..9cbeabac4f27 100644 --- a/arch/x86/net/bpf_jit_comp32.c +++ b/arch/x86/net/bpf_jit_comp32.c @@ -266,6 +266,51 @@ static inline void emit_ia32_mov_r64(const bool is64, const u8 dst[], emit_ia32_mov_i(dst_hi, 0, dstk, pprog); } +/* dst = sign_extend(src, insn->off) */ +static inline void emit_ia32_movsx_r64(const struct bpf_insn *insn, u8 **pprog, + const struct bpf_prog_aux *aux) +{ + const u8 *dst = bpf2ia32[insn->dst_reg]; + const u8 *src = bpf2ia32[insn->src_reg]; + bool dstk = insn->dst_reg != BPF_REG_AX; + bool sstk = insn->src_reg != BPF_REG_AX; + bool is64 = BPF_CLASS(insn->code) == BPF_ALU64; + u8 *prog = *pprog; + int cnt = 0; + + if (sstk) + /* mov eax,dword ptr [ebp+off] */ + EMIT3(0x8B, add_2reg(0x40, IA32_EBP, IA32_EAX), + STACK_VAR(src_lo)); + else + /* mov eax,src_lo */ + EMIT2(0x89, add_2reg(0xC0, IA32_EAX, src_lo)); + + switch (insn->off) { + case 8: + /* movsx eax,al */ + EMIT3(0x0F, 0xBE, 0xC0); + break; + case 16: + /* movsx eax,ax */ + EMIT3(0x0F, 0xBF, 0xC0); + break; + case 32: + /* EAX already holds the low word; CDQ supplies the high word. */ + break; + } + + emit_ia32_mov_r(dst_lo, IA32_EAX, dstk, false, &prog); + if (is64) { + EMIT1(0x99); /* cdq */ + emit_ia32_mov_r(dst_hi, IA32_EDX, dstk, false, &prog); + } else if (!aux->verifier_zext) { + emit_ia32_mov_i(dst_hi, 0, dstk, &prog); + } + + *pprog = prog; +} + /* Sign extended move */ static inline void emit_ia32_mov_i64(const bool is64, const u8 dst[], const u32 val, bool dstk, u8 **pprog) @@ -1697,6 +1742,12 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, emit_ia32_mov_i(dst_hi, 0, dstk, &prog); break; } + if (insn->off == 8 || insn->off == 16 || + (is64 && insn->off == 32)) { + emit_ia32_movsx_r64(insn, &prog, + bpf_prog->aux); + break; + } emit_ia32_mov_r64(is64, dst, src, dstk, sstk, &prog, bpf_prog->aux); break;