On Fri, Apr 11, 2025 at 1:32 PM Alexis Lothoré (eBPF Foundation) alexis.lothore@bootlin.com wrote:
In order to properly JIT the trampolines needed to attach BPF programs to functions, some architectures like ARM64 need to know about the alignment needed for the function arguments. Such alignment can generally be deduced from the argument size, but that's not completely true for composite types. In the specific case of ARM64, the AAPCS64 ABI defines that a composite type which needs to be passed through stack must be aligned on the maximum between 8 and the largest alignment constraint of its first-level members. So the JIT compiler needs more information about the arguments to make sure to generate code that respects those alignment constraints.
For struct arguments, add information about the size of the largest first-level member in the struct btf_func_model to allow the JIT compiler to guess the needed alignment. The information is quite
I might be missing something, but how can the *size* of the field be used to calculate that argument's *alignment*? i.e., I don't understand why arg_largest_member_size needs to be calculated instead of arg_largest_member_alignment...
specific, but it allows to keep arch-specific concerns (ie: guessing the final needed alignment for an argument) isolated in each JIT compiler.
couldn't all this information be calculated in the JIT compiler (if JIT needs that) from BTF?
Signed-off-by: Alexis Lothoré (eBPF Foundation) alexis.lothore@bootlin.com
include/linux/bpf.h | 1 + kernel/bpf/btf.c | 25 +++++++++++++++++++++++++ 2 files changed, 26 insertions(+)
diff --git a/include/linux/bpf.h b/include/linux/bpf.h index 3f0cc89c0622cb1a097999afb78c17102593b6bb..8b34dcf60a0ce09228ff761b962ab67b6a3e2263 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -1106,6 +1106,7 @@ struct btf_func_model { u8 nr_args; u8 arg_size[MAX_BPF_FUNC_ARGS]; u8 arg_flags[MAX_BPF_FUNC_ARGS];
u8 arg_largest_member_size[MAX_BPF_FUNC_ARGS];};
/* Restore arguments before returning from trampoline to let original function diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c index 16ba36f34dfab7531babf5753cab9f368cddefa3..5d40911ec90210086a6175d569abb6e52d75ad17 100644 --- a/kernel/bpf/btf.c +++ b/kernel/bpf/btf.c @@ -7318,6 +7318,29 @@ static int __get_type_size(struct btf *btf, u32 btf_id, return -EINVAL; }
+static u8 __get_largest_member_size(struct btf *btf, const struct btf_type *t) +{
const struct btf_member *member;const struct btf_type *mtype;u8 largest_member_size = 0;int i;if (!__btf_type_is_struct(t))return largest_member_size;for_each_member(i, t, member) {mtype = btf_type_by_id(btf, member->type);while (mtype && btf_type_is_modifier(mtype))mtype = btf_type_by_id(btf, mtype->type);if (!mtype)return -EINVAL;if (mtype->size > largest_member_size)largest_member_size = mtype->size;}return largest_member_size;+}
static u8 __get_type_fmodel_flags(const struct btf_type *t) { u8 flags = 0; @@ -7396,6 +7419,8 @@ int btf_distill_func_proto(struct bpf_verifier_log *log, } m->arg_size[i] = ret; m->arg_flags[i] = __get_type_fmodel_flags(t);
m->arg_largest_member_size[i] =__get_largest_member_size(btf, t); } m->nr_args = nargs; return 0;-- 2.49.0