[PATCH v2 2/5] powerpc/bpf: Move out dummy_tramp_addr after Long branch stub
From: <hidden>
Date: 2026-04-02 20:40:41
Also in:
linux-kselftest, linuxppc-dev, 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 | 35 ++++++++++++++++++++++----------- 1 file changed, 24 insertions(+), 11 deletions(-)
diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
index 3cbb3647f7a0..00abad48fa6c 100644
--- a/arch/powerpc/net/bpf_jit_comp.c
+++ b/arch/powerpc/net/bpf_jit_comp.c@@ -93,27 +93,36 @@ 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,$+4 - * mflr r12 - * ld r12, -8-SZL(r12) + * mflr r12 // lr/r12 stores current pc + * ld r12, 20(r12) // offset(dummy_tramp_addr) from prev inst. is 20 * mtctr r12 - * mtlr r11 // needed to retain ftrace ABI + * mtlr r11 // needed to retain ftrace ABI * bctr + * nop // for alignment of following address field + * .long <dummy_tramp_addr> // 8-byte aligned */ - if (image) - *((unsigned long *)&image[ctx->idx]) = (unsigned long)dummy_tramp; - - ctx->idx += SZL / 4; long_branch_stub_idx = ctx->idx; EMIT(PPC_RAW_MFLR(_R11)); EMIT(PPC_RAW_BCL4()); EMIT(PPC_RAW_MFLR(_R12)); - EMIT(PPC_RAW_LL(_R12, _R12, -8-SZL)); + EMIT(PPC_RAW_LL(_R12, _R12, 20)); EMIT(PPC_RAW_MTCTR(_R12)); EMIT(PPC_RAW_MTLR(_R11)); EMIT(PPC_RAW_BCTR()); + /* + * The start of Long branch stub is guaranteed to be aligned as + * result of optional NOP injection before OOL stub above. + * Append tail NOP to re-gain 8-byte alignment disturbed by odd + * instruction count in Long branch stub. + */ + EMIT(PPC_RAW_NOP()); + + if (image) + *((unsigned long *)&image[ctx->idx]) = (unsigned long)dummy_tramp; + + ctx->idx += SZL / 4; if (!bpf_jit_ool_stub) { bpf_jit_ool_stub = (ctx->idx - ool_stub_idx) * 4;
@@ -1309,6 +1318,7 @@ static void do_isync(void *info __maybe_unused) * bpf_func: * [nop|b] ool_stub * 2. Out-of-line stub: + * nop // optional nop for alignment * ool_stub: * mflr r0 * [b|bl] <bpf_prog>/<long_branch_stub>
@@ -1316,7 +1326,6 @@ static void do_isync(void *info __maybe_unused) * b bpf_func + 4 * 3. Long branch stub: * long_branch_stub: - * .long <branch_addr>/<dummy_tramp> * mflr r11 * bcl 20,31,$+4 * mflr r12
@@ -1324,6 +1333,8 @@ static void do_isync(void *info __maybe_unused) * mtctr r12 * mtlr r11 // needed to retain ftrace ABI * bctr + * nop // nop for mem alignment of dummy_tramp_addr + * .long <branch_addr>/<dummy_tramp> * * dummy_tramp is used to reduce synchronization requirements. *
@@ -1425,10 +1436,12 @@ 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)
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2.52.0