Re: [PATCH 1/2] bpf, arm64: remove structs on stack constraint

["Alexis Lothoré" <alexis.lothore@bootlin.com>]

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-v1-0-5be9211768c3@bootlin.com/
>> [2] https://lore.kernel.org/bpf/CAADnVQ+sj9XhscN9PdmTzjVa7Eif21noAUH3y1K6x5bWcL-5pg@mail.gmail.com/
>> [3] https://lore.kernel.org/bpf/20250707-btf_skip_structs_on_stack-v3-0-29569e086c12@bootlin.com/
>> 
>> 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-5pg@mail.gmail.com/


-- 
Alexis Lothoré, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com