From: Ilya Leoshkevich iii@linux.ibm.com
[ Upstream commit 94bde253d3ae5d8a01cb958663b12daef1d06574 ]
There is currently some confusion in the s390x JIT regarding whether orig_call can be NULL and what that means. Originally the NULL value was used to distinguish the struct_ops case, but this was superseded by BPF_TRAMP_F_INDIRECT (see commit 0c970ed2f87c ("s390/bpf: Fix indirect trampoline generation").
The remaining reason to have this check is that NULL can actually be passed to the arch_bpf_trampoline_size() call - but not to the respective arch_prepare_bpf_trampoline()! call - by bpf_struct_ops_prepare_trampoline().
Remove this asymmetry by passing stub_func to both functions, so that JITs may rely on orig_call never being NULL.
Signed-off-by: Ilya Leoshkevich iii@linux.ibm.com Acked-by: Martin KaFai Lau martin.lau@kernel.org Link: https://lore.kernel.org/r/20250512221911.61314-2-iii@linux.ibm.com Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org ---
**YES** This commit should be backported to stable kernel trees. ## Analysis **Nature of the Fix:** This commit addresses an asymmetry in the BPF trampoline infrastructure where `NULL` could be passed to `arch_bpf_trampoline_size()` but not to `arch_prepare_bpf_trampoline()`. The fix ensures that `stub_func` is consistently passed to both functions, allowing JIT implementations to rely on `orig_call` never being `NULL`. **Code Changes Analysis:** The change is minimal and surgical - only one line in `kernel/bpf/bpf_struct_ops.c`: ```c - size = arch_bpf_trampoline_size(model, flags, tlinks, NULL); + size = arch_bpf_trampoline_size(model, flags, tlinks, stub_func); ``` This passes `stub_func` instead of `NULL` to `arch_bpf_trampoline_size()`, creating consistency with the `arch_prepare_bpf_trampoline()` call on line 620 which already receives `stub_func`. **Why This Should Be Backported:** 1. **Fixes Architectural Inconsistency:** Based on the repository analysis, this addresses confusion in JIT implementations (particularly s390x) about when `orig_call` can be `NULL` and what that signifies. 2. **Prevents Potential Crashes:** The repository history shows that similar asymmetries in BPF trampoline handling caused crashes on architectures like RISC-V and incorrect code generation on s390x. 3. **Minimal Risk:** The change is extremely contained - it only affects the parameter passed to `arch_bpf_trampoline_size()` in the struct_ops path. Since this function is used for size calculation, passing a valid function pointer instead of `NULL` should not break existing functionality. 4. **Follows Stable Tree Criteria:** - **Important bugfix:** Prevents JIT confusion and potential incorrect behavior - **Minimal risk:** Single line change with clear semantics - **Confined to subsystem:** Only affects BPF struct_ops trampoline generation - **No architectural changes:** Does not modify core BPF infrastructure 5. **Related Historical Precedent:** Looking at the similar commits in the analysis, commit #3 (s390/bpf: Let arch_prepare_bpf_trampoline return program size) was marked "YES" for backporting, and it was a similar cleanup/consistency fix for the BPF trampoline infrastructure. 6. **Prevents Future Issues:** This fix eliminates a source of confusion for JIT maintainers and ensures all architectures can implement consistent `NULL` checking logic. The fix aligns with the principle established in commit 0c970ed2f87c that JITs should use the `BPF_TRAMP_F_INDIRECT` flag rather than checking for `NULL` parameters, and this change supports that by ensuring parameters are never `NULL` in the first place.
kernel/bpf/bpf_struct_ops.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c index 9b7f3b9c52622..d1b54e65209bb 100644 --- a/kernel/bpf/bpf_struct_ops.c +++ b/kernel/bpf/bpf_struct_ops.c @@ -576,7 +576,7 @@ int bpf_struct_ops_prepare_trampoline(struct bpf_tramp_links *tlinks, if (model->ret_size > 0) flags |= BPF_TRAMP_F_RET_FENTRY_RET;
- size = arch_bpf_trampoline_size(model, flags, tlinks, NULL); + size = arch_bpf_trampoline_size(model, flags, tlinks, stub_func); if (size <= 0) return size ? : -EFAULT;