Hello, this series follows up on the one introducing 9+ args for tracing programs [1]. It has been observed with this series that there are cases for which we can not identify accurately the location of the target function arguments to prepare correctly the corresponding BPF trampoline. This is the case for example if: - the function consumes a struct variable _by value_ - it is passed on the stack (no more register available for it) - it has some __packed__ or __aligned(X)__ attribute
As a consequence, a small restrictive check has been added to the ARM64 side, highlighting that other arch supporting 9+ args in BPF trampolines are already suffering from the same issue. After a bit of discussions and attempts, the chosen solution is, rather than applying the same constraint to all JIT compilers, to prevent such function from being encoded at all in BTF info([2]). As the pahole side is closed to be integrated, we can now remove the restrictive check from kernel side.
[1] https://lore.kernel.org/bpf/20250527-many_args_arm64-v3-0-3faf7bb8e4a2@bootl... [2] https://lore.kernel.org/bpf/20250707-btf_skip_structs_on_stack-v3-0-29569e08...
Signed-off-by: Alexis Lothoré (eBPF Foundation) alexis.lothore@bootlin.com --- Alexis Lothoré (eBPF Foundation) (2): bpf, arm64: remove structs on stack constraint selftests/bpf: enable tracing_struct tests for arm64
arch/arm64/net/bpf_jit_comp.c | 5 ----- tools/testing/selftests/bpf/DENYLIST.aarch64 | 1 - 2 files changed, 6 deletions(-) --- base-commit: 8da1e37fc84868b50ba6a7cdf082aa3b0d11e006 change-id: 20250708-arm64_relax_jit_comp-e8889647d8d2
Best regards,
While introducing support for 9+ arguments for tracing programs on ARM64, commit 9014cf56f13d ("bpf, arm64: Support up to 12 function arguments") has also introduced a constraint preventing BPF trampolines from being generated if the target function consumes a struct argument passed on stack, because of uncertainties around the exact struct location: if the struct has been marked as packed or with a custom alignment, this info is not reflected in BTF data, and so generated tracing trampolines could read the target function arguments at wrong offsets.
This issue is not specific to ARM64: there has been an attempt (see [1]) to bring the same constraint to other architectures JIT compilers. But discussions following this attempt led to the move of this constraint out of the kernel (see [2]): instead of preventing the kernel from generating trampolines for those functions consuming structs on stack, it is simpler to just make sure that those functions with uncertain struct arguments location are not encoded in BTF information, and so that one can not even attempt to attach a tracing program to such function. The task is then deferred to pahole (see [3]).
Now that the constraint is handled by pahole, remove it from the arm64 JIT compiler to keep it simple.
[1] https://lore.kernel.org/bpf/20250613-deny_trampoline_structs_on_stack-v1-0-5... [2] https://lore.kernel.org/bpf/CAADnVQ+sj9XhscN9PdmTzjVa7Eif21noAUH3y1K6x5bWcL-... [3] https://lore.kernel.org/bpf/20250707-btf_skip_structs_on_stack-v3-0-29569e08...
Signed-off-by: Alexis Lothoré (eBPF Foundation) alexis.lothore@bootlin.com --- arch/arm64/net/bpf_jit_comp.c | 5 ----- 1 file changed, 5 deletions(-)
diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c index b6c42b5c96688251ea24f5e771fa1effff896541..89b1b8c248c62e09cec61e13318d45b59006dce1 100644 --- a/arch/arm64/net/bpf_jit_comp.c +++ b/arch/arm64/net/bpf_jit_comp.c @@ -2240,11 +2240,6 @@ static int calc_arg_aux(const struct btf_func_model *m,
/* the rest arguments are passed through stack */ for (; i < m->nr_args; i++) { - /* We can not know for sure about exact alignment needs for - * struct passed on stack, so deny those - */ - if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG) - return -ENOTSUPP; stack_slots = (m->arg_size[i] + 7) / 8; a->bstack_for_args += stack_slots * 8; a->ostack_for_args = a->ostack_for_args + stack_slots * 8;
On Wed, Jul 09, 2025 at 10:36:55AM +0200, Alexis Lothoré (eBPF Foundation) wrote:
While introducing support for 9+ arguments for tracing programs on ARM64, commit 9014cf56f13d ("bpf, arm64: Support up to 12 function arguments") has also introduced a constraint preventing BPF trampolines from being generated if the target function consumes a struct argument passed on stack, because of uncertainties around the exact struct location: if the struct has been marked as packed or with a custom alignment, this info is not reflected in BTF data, and so generated tracing trampolines could read the target function arguments at wrong offsets.
This issue is not specific to ARM64: there has been an attempt (see [1]) to bring the same constraint to other architectures JIT compilers. But discussions following this attempt led to the move of this constraint out of the kernel (see [2]): instead of preventing the kernel from generating trampolines for those functions consuming structs on stack, it is simpler to just make sure that those functions with uncertain struct arguments location are not encoded in BTF information, and so that one can not even attempt to attach a tracing program to such function. The task is then deferred to pahole (see [3]).
Now that the constraint is handled by pahole, remove it from the arm64 JIT compiler to keep it simple.
[1] https://lore.kernel.org/bpf/20250613-deny_trampoline_structs_on_stack-v1-0-5... [2] https://lore.kernel.org/bpf/CAADnVQ+sj9XhscN9PdmTzjVa7Eif21noAUH3y1K6x5bWcL-... [3] https://lore.kernel.org/bpf/20250707-btf_skip_structs_on_stack-v3-0-29569e08...
Signed-off-by: Alexis Lothoré (eBPF Foundation) alexis.lothore@bootlin.com
arch/arm64/net/bpf_jit_comp.c | 5 ----- 1 file changed, 5 deletions(-)
This is a question born more out of ignorance that insight, but how do we ensure that the version of pahole being used is sufficiently up-to-date that the in-kernel check is not required?
Will
On Tue Jul 15, 2025 at 3:32 PM CEST, Will Deacon wrote:
On Wed, Jul 09, 2025 at 10:36:55AM +0200, Alexis Lothoré (eBPF Foundation) wrote:
While introducing support for 9+ arguments for tracing programs on ARM64, commit 9014cf56f13d ("bpf, arm64: Support up to 12 function arguments") has also introduced a constraint preventing BPF trampolines from being generated if the target function consumes a struct argument passed on stack, because of uncertainties around the exact struct location: if the struct has been marked as packed or with a custom alignment, this info is not reflected in BTF data, and so generated tracing trampolines could read the target function arguments at wrong offsets.
This issue is not specific to ARM64: there has been an attempt (see [1]) to bring the same constraint to other architectures JIT compilers. But discussions following this attempt led to the move of this constraint out of the kernel (see [2]): instead of preventing the kernel from generating trampolines for those functions consuming structs on stack, it is simpler to just make sure that those functions with uncertain struct arguments location are not encoded in BTF information, and so that one can not even attempt to attach a tracing program to such function. The task is then deferred to pahole (see [3]).
Now that the constraint is handled by pahole, remove it from the arm64 JIT compiler to keep it simple.
[1] https://lore.kernel.org/bpf/20250613-deny_trampoline_structs_on_stack-v1-0-5... [2] https://lore.kernel.org/bpf/CAADnVQ+sj9XhscN9PdmTzjVa7Eif21noAUH3y1K6x5bWcL-... [3] https://lore.kernel.org/bpf/20250707-btf_skip_structs_on_stack-v3-0-29569e08...
Signed-off-by: Alexis Lothoré (eBPF Foundation) alexis.lothore@bootlin.com
arch/arm64/net/bpf_jit_comp.c | 5 ----- 1 file changed, 5 deletions(-)
This is a question born more out of ignorance that insight, but how do we ensure that the version of pahole being used is sufficiently up-to-date that the in-kernel check is not required?
Based on earlier discussions, I am not convinced it is worth maintaining the check depending on the pahole version used in BTF. Other architectures exposing a JIT compiler don't have the in-kernel check and so are already exposed to this very specific case, but discussions around my attempt to enforce the check on other JIT comp showed that the rarity of this case do not justify protecting it on kernel side (see [1]).
Alexis
[1] https://lore.kernel.org/bpf/CAADnVQ+sj9XhscN9PdmTzjVa7Eif21noAUH3y1K6x5bWcL-...
On Tue, Jul 15, 2025 at 04:02:25PM +0200, Alexis Lothoré wrote:
On Tue Jul 15, 2025 at 3:32 PM CEST, Will Deacon wrote:
On Wed, Jul 09, 2025 at 10:36:55AM +0200, Alexis Lothoré (eBPF Foundation) wrote:
While introducing support for 9+ arguments for tracing programs on ARM64, commit 9014cf56f13d ("bpf, arm64: Support up to 12 function arguments") has also introduced a constraint preventing BPF trampolines from being generated if the target function consumes a struct argument passed on stack, because of uncertainties around the exact struct location: if the struct has been marked as packed or with a custom alignment, this info is not reflected in BTF data, and so generated tracing trampolines could read the target function arguments at wrong offsets.
This issue is not specific to ARM64: there has been an attempt (see [1]) to bring the same constraint to other architectures JIT compilers. But discussions following this attempt led to the move of this constraint out of the kernel (see [2]): instead of preventing the kernel from generating trampolines for those functions consuming structs on stack, it is simpler to just make sure that those functions with uncertain struct arguments location are not encoded in BTF information, and so that one can not even attempt to attach a tracing program to such function. The task is then deferred to pahole (see [3]).
Now that the constraint is handled by pahole, remove it from the arm64 JIT compiler to keep it simple.
[1] https://lore.kernel.org/bpf/20250613-deny_trampoline_structs_on_stack-v1-0-5... [2] https://lore.kernel.org/bpf/CAADnVQ+sj9XhscN9PdmTzjVa7Eif21noAUH3y1K6x5bWcL-... [3] https://lore.kernel.org/bpf/20250707-btf_skip_structs_on_stack-v3-0-29569e08...
Signed-off-by: Alexis Lothoré (eBPF Foundation) alexis.lothore@bootlin.com
arch/arm64/net/bpf_jit_comp.c | 5 ----- 1 file changed, 5 deletions(-)
This is a question born more out of ignorance that insight, but how do we ensure that the version of pahole being used is sufficiently up-to-date that the in-kernel check is not required?
Based on earlier discussions, I am not convinced it is worth maintaining the check depending on the pahole version used in BTF. Other architectures exposing a JIT compiler don't have the in-kernel check and so are already exposed to this very specific case, but discussions around my attempt to enforce the check on other JIT comp showed that the rarity of this case do not justify protecting it on kernel side (see [1]).
I can understand why doing this in pahole rather than in each individual JIT is preferable, but I don't think there's any harm leaving the existing two line check in arm64 as long as older versions of pahole might be used, is there? I wouldn't say that removing it really simplifies the JIT compiler when you consider the rest of the implementation.
Of course, once the kernel requires a version of pahole recent enough to contain [3], we should drop the check in the JIT compiler as the one in pahole looks like it's more selective about the functions it rejects.
Will
On Tue, Jul 15, 2025 at 7:31 AM Will Deacon will@kernel.org wrote:
On Tue, Jul 15, 2025 at 04:02:25PM +0200, Alexis Lothoré wrote:
On Tue Jul 15, 2025 at 3:32 PM CEST, Will Deacon wrote:
On Wed, Jul 09, 2025 at 10:36:55AM +0200, Alexis Lothoré (eBPF Foundation) wrote:
While introducing support for 9+ arguments for tracing programs on ARM64, commit 9014cf56f13d ("bpf, arm64: Support up to 12 function arguments") has also introduced a constraint preventing BPF trampolines from being generated if the target function consumes a struct argument passed on stack, because of uncertainties around the exact struct location: if the struct has been marked as packed or with a custom alignment, this info is not reflected in BTF data, and so generated tracing trampolines could read the target function arguments at wrong offsets.
This issue is not specific to ARM64: there has been an attempt (see [1]) to bring the same constraint to other architectures JIT compilers. But discussions following this attempt led to the move of this constraint out of the kernel (see [2]): instead of preventing the kernel from generating trampolines for those functions consuming structs on stack, it is simpler to just make sure that those functions with uncertain struct arguments location are not encoded in BTF information, and so that one can not even attempt to attach a tracing program to such function. The task is then deferred to pahole (see [3]).
Now that the constraint is handled by pahole, remove it from the arm64 JIT compiler to keep it simple.
[1] https://lore.kernel.org/bpf/20250613-deny_trampoline_structs_on_stack-v1-0-5... [2] https://lore.kernel.org/bpf/CAADnVQ+sj9XhscN9PdmTzjVa7Eif21noAUH3y1K6x5bWcL-... [3] https://lore.kernel.org/bpf/20250707-btf_skip_structs_on_stack-v3-0-29569e08...
Signed-off-by: Alexis Lothoré (eBPF Foundation) alexis.lothore@bootlin.com
arch/arm64/net/bpf_jit_comp.c | 5 ----- 1 file changed, 5 deletions(-)
This is a question born more out of ignorance that insight, but how do we ensure that the version of pahole being used is sufficiently up-to-date that the in-kernel check is not required?
Based on earlier discussions, I am not convinced it is worth maintaining the check depending on the pahole version used in BTF. Other architectures exposing a JIT compiler don't have the in-kernel check and so are already exposed to this very specific case, but discussions around my attempt to enforce the check on other JIT comp showed that the rarity of this case do not justify protecting it on kernel side (see [1]).
I can understand why doing this in pahole rather than in each individual JIT is preferable, but I don't think there's any harm leaving the existing two line check in arm64 as long as older versions of pahole might be used, is there? I wouldn't say that removing it really simplifies the JIT compiler when you consider the rest of the implementation.
Of course, once the kernel requires a version of pahole recent enough to contain [3], we should drop the check in the JIT compiler as the one in pahole looks like it's more selective about the functions it rejects.
I frankly don't see the point in adding and maintaining such checks and code in the kernel for hypothetical cases that are not present in the kernel and highly unlikely ever be. The arm64 jit check was added out of abundance of caution. There was way too much "caution".
On Tue Jul 15, 2025 at 5:40 PM CEST, Alexei Starovoitov wrote:
On Tue, Jul 15, 2025 at 7:31 AM Will Deacon will@kernel.org wrote:
On Tue, Jul 15, 2025 at 04:02:25PM +0200, Alexis Lothoré wrote:
On Tue Jul 15, 2025 at 3:32 PM CEST, Will Deacon wrote:
On Wed, Jul 09, 2025 at 10:36:55AM +0200, Alexis Lothoré (eBPF Foundation) wrote:
While introducing support for 9+ arguments for tracing programs on ARM64, commit 9014cf56f13d ("bpf, arm64: Support up to 12 function arguments") has also introduced a constraint preventing BPF trampolines from being generated if the target function consumes a struct argument passed on stack, because of uncertainties around the exact struct location: if the struct has been marked as packed or with a custom alignment, this info is not reflected in BTF data, and so generated tracing trampolines could read the target function arguments at wrong offsets.
This issue is not specific to ARM64: there has been an attempt (see [1]) to bring the same constraint to other architectures JIT compilers. But discussions following this attempt led to the move of this constraint out of the kernel (see [2]): instead of preventing the kernel from generating trampolines for those functions consuming structs on stack, it is simpler to just make sure that those functions with uncertain struct arguments location are not encoded in BTF information, and so that one can not even attempt to attach a tracing program to such function. The task is then deferred to pahole (see [3]).
Now that the constraint is handled by pahole, remove it from the arm64 JIT compiler to keep it simple.
[1] https://lore.kernel.org/bpf/20250613-deny_trampoline_structs_on_stack-v1-0-5... [2] https://lore.kernel.org/bpf/CAADnVQ+sj9XhscN9PdmTzjVa7Eif21noAUH3y1K6x5bWcL-... [3] https://lore.kernel.org/bpf/20250707-btf_skip_structs_on_stack-v3-0-29569e08...
Signed-off-by: Alexis Lothoré (eBPF Foundation) alexis.lothore@bootlin.com
arch/arm64/net/bpf_jit_comp.c | 5 ----- 1 file changed, 5 deletions(-)
This is a question born more out of ignorance that insight, but how do we ensure that the version of pahole being used is sufficiently up-to-date that the in-kernel check is not required?
Based on earlier discussions, I am not convinced it is worth maintaining the check depending on the pahole version used in BTF. Other architectures exposing a JIT compiler don't have the in-kernel check and so are already exposed to this very specific case, but discussions around my attempt to enforce the check on other JIT comp showed that the rarity of this case do not justify protecting it on kernel side (see [1]).
I can understand why doing this in pahole rather than in each individual JIT is preferable, but I don't think there's any harm leaving the existing two line check in arm64 as long as older versions of pahole might be used, is there? I wouldn't say that removing it really simplifies the JIT compiler when you consider the rest of the implementation.
Of course, once the kernel requires a version of pahole recent enough to contain [3], we should drop the check in the JIT compiler as the one in pahole looks like it's more selective about the functions it rejects.
I frankly don't see the point in adding and maintaining such checks and code in the kernel for hypothetical cases that are not present in the kernel and highly unlikely ever be. The arm64 jit check was added out of abundance of caution. There was way too much "caution".
To complete Alexei's point: the check currently implemented in ARM64 JIT comp is filtering _too many_ functions. It prevents attachment to any function consuming a struct passed by value on the stack. But what we really want is only about filtering those _when their alignment is altered_, which is something that can not currently be deduced at runtime in JIT comps. That's part of the reason why this has been moved to pahole.
I would also add that there is another small drawback about keeping this check on ARM64 only, while not adding it to other JIT comps: we have to keep filtering out some tests for ARM64, while the feature set is actually the same as for the other archs. Sure, this discrepancy could be eliminated once pahole minimum version in kernel build system contains the needed development, as Will suggests, but I don't see that hapenning before many months/years.
Alexis
Now that the constraint preventing attachment to functions consuming struct on stack has been removed from the kernel (and moved to pahole, with a slightly smarter detection, to prevent only those that are packed), re-enable the tracing_struct tests for arm64.
Signed-off-by: Alexis Lothoré (eBPF Foundation) alexis.lothore@bootlin.com --- tools/testing/selftests/bpf/DENYLIST.aarch64 | 1 - 1 file changed, 1 deletion(-)
diff --git a/tools/testing/selftests/bpf/DENYLIST.aarch64 b/tools/testing/selftests/bpf/DENYLIST.aarch64 deleted file mode 100644 index 12e99c0277a8cbf9e63e8f6d3a108c8a1208407b..0000000000000000000000000000000000000000 --- a/tools/testing/selftests/bpf/DENYLIST.aarch64 +++ /dev/null @@ -1 +0,0 @@ -tracing_struct/struct_many_args # struct_many_args:FAIL:tracing_struct_many_args__attach unexpected error: -524
Hello:
This series was applied to bpf/bpf-next.git (master) by Alexei Starovoitov ast@kernel.org:
On Wed, 09 Jul 2025 10:36:54 +0200 you wrote:
Hello, this series follows up on the one introducing 9+ args for tracing programs [1]. It has been observed with this series that there are cases for which we can not identify accurately the location of the target function arguments to prepare correctly the corresponding BPF trampoline. This is the case for example if:
- the function consumes a struct variable _by value_
- it is passed on the stack (no more register available for it)
- it has some __packed__ or __aligned(X)__ attribute
[...]
Here is the summary with links: - [1/2] bpf, arm64: remove structs on stack constraint https://git.kernel.org/bpf/bpf-next/c/dc704d0cfa43 - [2/2] selftests/bpf: enable tracing_struct tests for arm64 https://git.kernel.org/bpf/bpf-next/c/4a760d2d7aa6
You are awesome, thank you!
linux-kselftest-mirror@lists.linaro.org
-
Alexei Starovoitov -
Alexis Lothoré -
Alexis Lothoré (eBPF Foundation)
-
patchwork-bot+netdevbpf＠kernel.org -
Will Deacon