[PATCH v5 2/6] powerpc/bpf: Move out dummy_tramp_addr after Long branch stub
From: <hidden>
Date: 2026-05-19 19:38:23
Also in:
bpf, linux-kselftest, stable
Subsystem:
bpf jit for powerpc (32-bit and 64-bit), bpf [general] (safe dynamic programs and tools), linux for powerpc (32-bit and 64-bit), the rest · Maintainers:
Hari Bathini, Christophe Leroy, Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, Madhavan Srinivasan, Michael Ellerman, Linus Torvalds
From: Abhishek Dubey <redacted> Move the long branch address space to the bottom of the long branch stub. This allows uninterrupted disassembly until the last 8 bytes. Exclude these last bytes from the overall program length to prevent failure in assembly generation. Also, align dummy_tramp_addr field with 8-byte boundary. Following is disassembler output for test program with moved down dummy_tramp_addr field: ..... ..... pc:68 left:44 a6 03 08 7c : mtlr 0 pc:72 left:40 bc ff ff 4b : b .-68 pc:76 left:36 a6 02 68 7d : mflr 11 pc:80 left:32 05 00 9f 42 : bcl 20, 31, .+4 pc:84 left:28 a6 02 88 7d : mflr 12 pc:88 left:24 14 00 8c e9 : ld 12, 20(12) pc:92 left:20 a6 03 89 7d : mtctr 12 pc:96 left:16 a6 03 68 7d : mtlr 11 pc:100 left:12 20 04 80 4e : bctr pc:104 left:8 c0 34 1d 00 : Failure log: Can't disasm instruction at offset 104: c0 34 1d 00 00 00 00 c0 Disassembly logic can truncate at 104, ignoring last 8 bytes. Update the dummy_tramp_addr field offset calculation from the end of the program to reflect its new location, for bpf_arch_text_poke() to update the actual trampoline's address in this field. All BPF trampoline selftests continue to pass with this patch applied. Signed-off-by: Abhishek Dubey <redacted> --- arch/powerpc/net/bpf_jit_comp.c | 41 ++++++++++++++++++--------------- 1 file changed, 23 insertions(+), 18 deletions(-)
diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
index 00b86ed97cb5..56a923d3908e 100644
--- a/arch/powerpc/net/bpf_jit_comp.c
+++ b/arch/powerpc/net/bpf_jit_comp.c@@ -52,9 +52,10 @@ asm ( void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context *ctx) { int ool_stub_idx, long_branch_stub_idx; - int ool_instrs; + int stubs_instrs; /* + * The dummy_tramp_addr field is placed at bottom of Long branch stub. * In the final pass, align the mis-aligned dummy_tramp_addr field * in the fimage. The alignment NOP must appear before OOL stub, * to make ool_stub_idx & long_branch_stub_idx constant from end.
@@ -62,11 +63,11 @@ void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context * dummy_tramp_addr must be 8-byte aligned for load-register * compatibility. Since fimage is guaranteed >= 8-byte aligned * by the allocator, alignment depends only on the instruction - * count offset. The OOL stub has 4 instructions (with - * CONFIG_PPC_FTRACE_OUT_OF_LINE) or 3 instructions (without) + * count offset. The stubs block has 11 instructions (with + * CONFIG_PPC_FTRACE_OUT_OF_LINE) or 10 instructions (without) * before dummy_tramp_addr. * - * Emit a NOP here if (ctx->idx + ool_instrs) is odd, so that + * Emit a NOP here if (ctx->idx + stubs_instrs) is odd, so that * dummy_tramp_addr lands at an even instruction offset (== 8-byte * aligned from an 8-byte aligned base). *
@@ -76,8 +77,8 @@ void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context * jited_len signifies correct program size. */ - ool_instrs = IS_ENABLED(CONFIG_PPC_FTRACE_OUT_OF_LINE) ? 4 : 3; - if (!image || ((ctx->idx + ool_instrs) & 1)) + stubs_instrs = IS_ENABLED(CONFIG_PPC_FTRACE_OUT_OF_LINE) ? 11 : 10; + if (!image || ((ctx->idx + stubs_instrs) & 1)) EMIT(PPC_RAW_NOP()); /* nop // optional, for alignment of dummy_tramp_addr
@@ -97,28 +98,29 @@ void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context /* * Long branch stub: - * .long <dummy_tramp_addr> // 8-byte aligned * mflr r11 * bcl 20,31,$+4 - * mflr r12 - * ld r12, -8-SZL(r12) + * mflr r12 // lr/r12 stores pc of current(this) inst. + * ld r12, 20(r12) // offset(dummy_tramp_addr) from prev inst. is 20 * mtctr r12 - * mtlr r11 // needed to retain ftrace ABI + * mtlr r11 // needed to retain ftrace ABI * bctr + * .long <dummy_tramp_addr> // 8-byte aligned */ - if (image) - *((unsigned long *)&image[ctx->idx]) = (unsigned long)dummy_tramp; - - ctx->idx += SZL / 4; long_branch_stub_idx = ctx->idx; EMIT(PPC_RAW_MFLR(_R11)); EMIT(PPC_RAW_BCL4()); EMIT(PPC_RAW_MFLR(_R12)); - EMIT(PPC_RAW_LL(_R12, _R12, -8-SZL)); + EMIT(PPC_RAW_LL(_R12, _R12, 20)); EMIT(PPC_RAW_MTCTR(_R12)); EMIT(PPC_RAW_MTLR(_R11)); EMIT(PPC_RAW_BCTR()); + if (image) + *((unsigned long *)&image[ctx->idx]) = (unsigned long)dummy_tramp; + + ctx->idx += SZL / 4; + if (!bpf_jit_ool_stub) { bpf_jit_ool_stub = (ctx->idx - ool_stub_idx) * 4; bpf_jit_long_branch_stub = (ctx->idx - long_branch_stub_idx) * 4;
@@ -1288,6 +1290,7 @@ static void do_isync(void *info __maybe_unused) * bpf_func: * [nop|b] ool_stub * 2. Out-of-line stub: + * nop // optional nop for alignment * ool_stub: * mflr r0 * [b|bl] <bpf_prog>/<long_branch_stub>
@@ -1295,14 +1298,14 @@ static void do_isync(void *info __maybe_unused) * b bpf_func + 4 * 3. Long branch stub: * long_branch_stub: - * .long <branch_addr>/<dummy_tramp> * mflr r11 * bcl 20,31,$+4 * mflr r12 - * ld r12, -16(r12) + * ld r12, 20(r12) * mtctr r12 * mtlr r11 // needed to retain ftrace ABI * bctr + * .long <branch_addr>/<dummy_tramp> * * dummy_tramp is used to reduce synchronization requirements. *
@@ -1404,10 +1407,12 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type old_t, * 1. Update the address in the long branch stub: * If new_addr is out of range, we will have to use the long branch stub, so patch new_addr * here. Otherwise, revert to dummy_tramp, but only if we had patched old_addr here. + * + * dummy_tramp_addr moved to bottom of long branch stub. */ if ((new_addr && !is_offset_in_branch_range(new_addr - ip)) || (old_addr && !is_offset_in_branch_range(old_addr - ip))) - ret = patch_ulong((void *)(bpf_func_end - bpf_jit_long_branch_stub - SZL), + ret = patch_ulong((void *)(bpf_func_end - SZL), /* SZL: dummy_tramp_addr offset */ (new_addr && !is_offset_in_branch_range(new_addr - ip)) ? (unsigned long)new_addr : (unsigned long)dummy_tramp); if (ret)
--
2.52.0