[PATCH v3 2/5] powerpc/bpf: Move out dummy_tramp_addr after Long branch stub

adubey
2026-04-11T18:14:26Z

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 <adubey@linux.ibm.com>
---
arch/powerpc/net/bpf_jit_comp.c | 42 +++++++++++++++++++--------------
1 file changed, 24 insertions(+), 18 deletions(-)

diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
index c255b30a37b0..c2ac4e355464 100644
--- a/arch/powerpc/net/bpf_jit_comp.c
+++ b/arch/powerpc/net/bpf_jit_comp.c
@@ -58,20 +58,22 @@ void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context
* in the fimage. The alignment NOP must appear before OOL stub,
* to make ool_stub_idx & long_branch_stub_idx constant from end.
*
+ * The dummy_tramp_addr field is placed at bottom of Long branch stub.
+ *
* Need alignment NOP in following conditions:
*
* OOL stub aligned CONFIG_PPC_FTRACE_OUT_OF_LINE Alignment NOP
- * Y Y N
- * Y N Y
- * N Y Y
- * N N N
+ * Y Y Y
+ * Y N N
+ * N Y N
+ * N N Y
*/
#ifdef CONFIG_PPC64
if (fimage && image) {
unsigned long pc = (unsigned long)fimage + CTX_NIA(ctx);

- if (IS_ALIGNED(pc, 8) ^
- IS_ENABLED(CONFIG_PPC_FTRACE_OUT_OF_LINE))
+ if (~(IS_ALIGNED(pc, 8) ^
+ IS_ENABLED(CONFIG_PPC_FTRACE_OUT_OF_LINE)))
EMIT(PPC_RAW_NOP());
}
#endif
@@ -93,28 +95,29 @@ 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;
@@ -1155,6 +1158,7 @@ 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>
@@ -1162,14 +1166,14 @@ 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.
*
@@ -1271,10 +1275,12 @@ 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