eddyz87 added a comment.
In D133361#4628951 <https://reviews.llvm.org/D133361#4628951>, @ast wrote:
> I agree that it's not useful outside of BPF, but it's useful outside of
> 'ctx'. I think 'preserve_constant_field_offset' would be more accurate
> description of the restriction.
> We can expand in the doc that it's a constant offset when field of the struct
> is accessed.
>
> Also instead of btf_tag it would be better to add another builtin similar to
> preserve_access_index.
> Currently we add __attribute__((preserve_access_index)) to trigger CO-RE.
> This one will be a new __attribute__((preserve_constant_field_offset)) that
> will be specified manually either in uapi/bpf.h or in vmlinux.h on some
> structs
This makes sense, I'll adjust the implementation to use new attribute (also
will try to use metadata instead of intrinsic call, replace by intrinsic on the
back-end side).
> and it will have precedence over preserve_access_index, so
> (__attribute__((preserve_access_index)), apply_to = record) in vmlinux.h will
> be ignored on such structs.
> Otherwise it's a bit odd that special names inside btf_tag have stronger
> rules than other __attribute__((preserve_access_index)).
Note on current propagation logic: whenever there is an expression E of type T,
where T is a struct annotated with btf_decl_tag("ctx"), all usages of E are
traversed recursively visiting `getelementptr` and calls to
`preserve_{struct,union,array}_index`, the latter are replaced by
`getelementptr`. E.g.:
#define __pai __attribute__((preserve_access_context));
#define __ctx __attribute__((btf_decl_tag("ctx")))
struct bar { int a; int b; } __pai;
struct foo {
struct bar b;
} __ctx __pai;
... struct foo f; ...
... f.b.bb ...
The call to `preserve_struct_index` generated for `bb` in `f.b.bb` would be
replaced by `getelementptr` and later by `getelementptr.and.load`.
However, context structures don't have nested structures at the moment. The
list of context structures is:
- __sk_buff
- bpf_cgroup_dev_ctx
- bpf_sk_lookup
- bpf_sock
- bpf_sock_addr
- bpf_sock_ops
- bpf_sockopt
- bpf_sysctl
- sk_msg_md
- sk_reuseport_md
- xdp_md
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D133361/new/
https://reviews.llvm.org/D133361
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
