Re: [PATCH v13 5/8] powerpc/bpf: Move out dummy_tramp_addr after Long branch stub
From: Hari Bathini <hbathini@linux.ibm.com>
Date: 2026-09-08 08:46:46
Also in:
bpf, linux-kselftest, stable
On 31/08/26 12:46 pm, Saket Kumar Bhaskar wrote:
From: Abhishek Dubey <redacted> Move the long branch address field to the bottom of the long branch stub. This allows uninterrupted disassembly until the last 4/8 bytes. The last bytes exclusion is logically necessary to prevent disassembly failure, otherwise the actual program layout is never altered. Hence no effect on overall program size. 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.
This makes it easy to disassemble the bpf programs. Looks good. Reviewed-by: Hari Bathini <hbathini@linux.ibm.com>
quoted hunk ↗ jump to hunk
Signed-off-by: Abhishek Dubey <redacted> Signed-off-by: Saket Kumar Bhaskar <redacted> Tested-by: Yeswanth Krishna Tellakula <redacted> --- arch/powerpc/net/bpf_jit_comp.c | 44 ++++++++++++++++++--------------- 1 file changed, 24 insertions(+), 20 deletions(-)diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c index 8ca36a933c7a..c2717f0d9cdd 100644 --- a/arch/powerpc/net/bpf_jit_comp.c +++ b/arch/powerpc/net/bpf_jit_comp.c@@ -52,19 +52,19 @@ 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_stub_sz; + int stub_sz; /* + * The dummy_tramp_addr field is placed at bottom of Long branch stub. * 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. * * The fimage can be non 8-byte aligned, so final alignment depends - * on start of fimage and the stub's instruction count offset. The - * OOL stub size is 4 instructions (with CONFIG_PPC_FTRACE_OUT_OF_LINE) - * or 3 instructions (without) before dummy_tramp_addr. - * - * Emit a NOP here if address is not SZL aligned. + * on start of fimage and the stub's instruction count. The + * stubs block has 11 instructions (with CONFIG_PPC_FTRACE_OUT_OF_LINE) + * or 10 instructions (without) before dummy_tramp_addr field. Emit a + * NOP if the address of dummy_tramp_addr is non aligned. * * In pass=0 when image==NULL, conservatively account for space * required to accommodate alignment NOP. In case final pass skips@@ -72,8 +72,8 @@ void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context * jited_len signifies correct program size. */ - ool_stub_sz = IS_ENABLED(CONFIG_PPC_FTRACE_OUT_OF_LINE) ? 16 : 12; - if (!image || !IS_ALIGNED((unsigned long)fimage + ctx->idx*4 + ool_stub_sz, SZL)) + stub_sz = IS_ENABLED(CONFIG_PPC_FTRACE_OUT_OF_LINE) ? 44 : 40; + if (!image || !IS_ALIGNED((unsigned long)fimage + ctx->idx*4 + stub_sz, SZL)) EMIT(PPC_RAW_NOP()); /*@@ -94,28 +94,29 @@ 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,$+4 - * mflr r12 - * ld r12, -8-SZL(r12) + * mflr r12 // lr/r12 stores pc of current(this) inst. + * ld r12, 20(r12) // offset(dummy_tramp_addr) from prev inst. is 20 * mtctr r12 - * mtlr r11 // needed to retain ftrace ABI + * mtlr r11 // needed to retain ftrace ABI * bctr + * .long <dummy_tramp_addr> // SZL bytes aligned */ - if (image) - *((unsigned long *)&image[ctx->idx]) = (unsigned long)dummy_tramp; - - ctx->idx += SZL / 4; long_branch_stub_idx = ctx->idx; EMIT(PPC_RAW_MFLR(_R11)); EMIT(PPC_RAW_BCL4()); EMIT(PPC_RAW_MFLR(_R12)); - EMIT(PPC_RAW_LL(_R12, _R12, -8-SZL)); + EMIT(PPC_RAW_LL(_R12, _R12, 20)); EMIT(PPC_RAW_MTCTR(_R12)); EMIT(PPC_RAW_MTLR(_R11)); EMIT(PPC_RAW_BCTR()); + if (image) + *((unsigned long *)&image[ctx->idx]) = (unsigned long)dummy_tramp; + + ctx->idx += SZL / 4; + if (!bpf_jit_ool_stub) { bpf_jit_ool_stub = (ctx->idx - ool_stub_idx) * 4; bpf_jit_long_branch_stub = (ctx->idx - long_branch_stub_idx) * 4;@@ -1309,6 +1310,7 @@ static void do_isync(void *info __maybe_unused) * bpf_func: * [nop|b] ool_stub * 2. Out-of-line stub: + * nop // optional nop for alignment * ool_stub: * mflr r0 * [b|bl] <bpf_prog>/<long_branch_stub>@@ -1316,14 +1318,14 @@ static void do_isync(void *info __maybe_unused) * b bpf_func + 4 * 3. Long branch stub: * long_branch_stub: - * .long <branch_addr>/<dummy_tramp> * mflr r11 * bcl 20,31,$+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. *@@ -1425,10 +1427,12 @@ 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)