On Fri, Jun 13, 2025 at 10:26:37AM +0200, Alexis Lothoré wrote: > Hi Peter, > > On Fri Jun 13, 2025 at 10:11 AM CEST, Peter Zijlstra wrote: > > On Fri, Jun 13, 2025 at 09:37:11AM +0200, Alexis Lothoré (eBPF Foundation) > > wrote: > >> When the target function receives more arguments than available > >> registers, the additional arguments are passed on stack, and so the > >> generated trampoline needs to read those to prepare the bpf context, > >> but also to prepare the target function stack when it is in charge of > >> calling it. This works well for scalar types, but if the value is a > >> struct, we can not know for sure the exact struct location, as it may > >> have been packed or manually aligned to a greater value. > > > > https://refspecs.linuxbase.org/elf/x86_64-abi-0.99.pdf > > > > Has fairly clear rules on how arguments are encoded. Broadly speaking > > for the kernel, if the structure exceeds 2 registers in size, it is > > passed as a reference, otherwise it is passed as two registers. > > Maybe my commit wording is not precise enough, but indeed, there's not > doubt about whether the struct value is passed on the stack or through a > register/a pair of registers. The doubt is rather about the struct location > when it is passed _by value_ and _on the stack_: the ABI indeed clearly > states that "Structures and unions assume the alignment of their most > strictly aligned component" (p.13), but this rule is "silently broken" when > a struct has an __attribute__((packed)) or and __attribute__((aligned(X))), > and AFAICT this case can not be detected at runtime with current BTF info.
Ah, okay. So it is a failure of BTF. That was indeed not clear.
