Em Mon, Mar 13, 2023 at 04:17:43AM +0200, Eduard Zingerman escreveu:
> In recent discussion in BPF mailing list ([1]) participants agreed to
> add a new DWARF representation for "btf_type_tag" annotations.
>
> Existing representation is DW_TAG_LLVM_annotation object attached as a
> child to a DW_TAG_pointer_type. It means that "btf_type_tag"
> annotation is attached to a pointee type.
>
> New representation is DW_TAG_LLVM_annotation object attached as a
> child to *any* type. It means that "btf_type_tag" annotation is
> attached to the parent type.
>
> Here is an example:
>
> int __attribute__((btf_type_tag("tag1"))) *g;
>
> And corresponding DWARF:
>
> 0x0000001e: DW_TAG_variable
> DW_AT_name ("g")
> DW_AT_type (0x00000029 "int *")
>
> 0x00000029: DW_TAG_pointer_type
> DW_AT_type (0x00000032 "int")
>
> 0x0000002e: DW_TAG_LLVM_annotation
> DW_AT_name ("btf_type_tag")
> DW_AT_const_value ("tag1")
>
> 0x00000032: DW_TAG_base_type
> DW_AT_name ("int")
>
>
> This patch adds logic necessary to handle such annotations in the
> pahole tool. Examples like below are supported:
>
> #define __tag(val) __attribute__((btf_type_tag("__" #val)))
>
> struct alpha {};
> union bravo {};
> enum charlie { X };
> typedef int delta;
>
> struct echo {
> int * __tag(a) a;
> int __tag(b) *b;
> int __tag(c) c;
> void __tag(d) *d;
> void __tag(e) *e;
> struct alpha __tag(f) f;
> union bravo __tag(g) g;
> enum charlie __tag(h) h;
> delta __tag(i) i;
> int __tag(j_result) (__tag(j) *j)(int __tag(j_param));
> } g;
>
> Implementation details
> ----------------------
>
> Although this was not discussed in the mailing list, the proposed
> implementation acts in a following way (for compatibility reasons):
> - both forms could be present in the debug info;
> - if any annotations corresponding to the new form are present in the
> debug info, annotations corresponding to the old form are ignored.
agreed
> Because the new form of type tags could be applied to any type it is
> somewhat invasive from implementation point of view:
Can you please see if you can break this patch into a series so that it
helps with reviewing and bisection?
> - Types `struct btf_type_tag_type` and `struct llvm_annotation` are
> consolidated as a single type `struct llvm_annotation`, in order to
> reside in a single `annots` list associated with struct/union/enum
> or typedef.
> - The `annots` list, used to hold references to annotation objects,
> is put directly in `struct tag`.
> - At the load phase it is not yet known whether new or old form is
> used for type tags encoding, so `struct llvm_annotation` objects are
> not added to the types table.
> - The recode phase now consists of several steps:
> - depending on the results of the load phase, `struct llvm_annotation`
> objects corresponding to either old or new representations are
> added to the types table;
> - `tag__recode_dwarf_type()` is executed as usual, but it also
> collects information about type tags;
> - references to types that have type tag annotations are updated to
> point to the first annotation object.
>
> Corresponding clang changes are tracked in [6].
>
> Testing
> -------
>
> To verify the changes I used the following:
> - Tools:
> - "LLVM-main" :: LLVM at revision [3];
> - "LLVM-new" :: LLVM at revision [3] with patches [6] applied;
> - "gcc" :: GCC version 11.3 (no support for btf_type_tag annotations);
> - "pahole-next" :: dwarves at revision [4];
> - "pahole-new" :: dwarves at revision [4] + this patch,
> - "kernel" :: Linux Kernel bpf-next branch at revision [5]
> - test cases:
> - kernel build;
> - kernel BPF test cases build, BPF tests execution
> (test_verifier, test_progs, test_progs-no_alu32, test_maps);
> - btfdiff script (suggested by Arnaldo, [2]).
> - tool combinations (kernel compiler / clang for BPF tests / pahole version):
> - LLVM-main / LLVM-main / pahole-new
> - kernel build : ok
> - bpf tests : ok
> - btfdiff : ok (modulo diff #1, see below)
> - gcc / LLVM-main / pahole-new
> - kernel build : ok
> - bpf tests : ok
> - btfdiff : ok
> - LLVM-new / LLVM-new / pahole-next
> - kernel build : ok with warnings (see warn #1 below)
> - bpf tests : ok
> - btfdiff : fails (see diff #2 below)
> - LLVM-new / LLVM-new / pahole-new
> - kernel build : ok
> - bpf tests : ok
> - btfdiff : ok (modulo diff #1, see below)
> - gcc / LLVM-new / pahole-new
> - kernel build : ok
> - bpf tests : ok
> - btfdiff : ok
>
> Note on BPF tests: test case `verif_scale_loop6` fails for all
> configurations above and 'LLVM-main / LLVM-main / pahole-next',
> thus I consider this issue as unrelated.
>
> Diff #1: Difference in flexible printing, several occurrences as below:
>
> @ -10531,7 +10531,7 @ struct bpf_cand_cache {
> struct {
> const struct btf * btf; /* 16 8 */
> u32 id; /* 24 4 */
> - } cands[0]; /* 16 0 */
> + } cands[]; /* 16 0 */
>
> /* size: 16, cachelines: 1, members: 5 */
> /* last cacheline: 16 bytes */
>
> Diff #2: pahole-next does not know how to print type name for types
> with type tags, when reading from BTF:
>
> @ -72998,8 +73022,8 @ struct sock {
> /* --- cacheline 19 boundary (1216 bytes) --- */
> int (*sk_backlog_rcv)(struct sock *, struct sk_buff *); /* 1216 8 */
> void (*sk_destruct)(struct sock *); /* 1224 8 */
> - rcu * sk_reuseport_cb; /* 1232 8 */
> - rcu * sk_bpf_storage; /* 1240 8 */
> + <ERROR > sk_reuseport_cb; /* 1232 8 */
> + <ERROR > sk_bpf_storage; /* 1240 8 */
>
> (Also DWARF names refer to TYPE_TAG, not actual type name, fixed in pahole-new).
>
> Warn #1: pahole-next complains about unexpected child tags generated
> by clang, e.g.:
>
> die__create_new_tag: unspecified_type WITH children!
> die__create_new_base_type: DW_TAG_base_type WITH children!
Sure, we can remove those if the children is the expected one, leaving
the warning maybe for debug sessions.
Thanks for the detailed implementation notes, references and tests
performed, please consider breaking it up into smaller pieces or
ellaborate on why you think can't be done.
Thanks!
- Arnaldo
>
> Performance impact
> ------------------
>
> The update to `struct tag` might raise concerns regarding memory
> usage, additional steps in recode phase might raise concerns regarding
> execution time. Below is statistics collected for Kernel BTF
> generation.
>
> LLVM-new / LLVM-new / pahole-new:
>
> $ /usr/bin/time -v pahole -J --btf_gen_floats -j --lang_exclude=rust .tmp_vmlinux.btf
> ...
> User time (seconds): 22.29
> System time (seconds): 0.47
> Percent of CPU this job got: 483%
> ...
> Maximum resident set size (kbytes): 714524
> ...
>
> LLVM-new / LLVM-new / pahole-next:
>
> $ /usr/bin/time -v pahole -J --btf_gen_floats -j --lang_exclude=rust .tmp_vmlinux.btf
> ...
> User time (seconds): 20.96
> System time (seconds): 0.44
> Percent of CPU this job got: 473%
> ...
> Maximum resident set size (kbytes): 700848
> ...
>
> Links & revisions
> -----------------
>
> [1] Mailing list discussion regarding `btf:type_tag`
> https://lore.kernel.org/bpf/87r0w9jjoq.fsf@xxxxxxxxxx/
> [2] Suggestion to use btfdiff
> https://lore.kernel.org/dwarves/ZAKpZGSHTvsS4r8E@xxxxxxxxxx/T/#mddbfe661e339485fb2b0e706b31329b46bf61bda
> [3] f759275c1c8e ("[AMDGPU] Regenerate sdwa-peephole.ll")
> [4] a9498899109d ("dwarf_loader: Support for btf:type_tag")
> [5] 49b5300f1f8f ("Merge branch 'Support stashing local kptrs with bpf_kptr_xchg'")
> [6] LLVM changes to generate btf:type_tag, revisions stack:
> https://reviews.llvm.org/D143966
> https://reviews.llvm.org/D143967
> https://reviews.llvm.org/D145891
>
> Eduard Zingerman (1):
> dwarf_loader: Support for btf:type_tag
>
> btf_encoder.c | 13 +-
> btf_loader.c | 15 +-
> dwarf_loader.c | 763 +++++++++++++++++++++++++++++++++++++---------
> dwarves.c | 1 +
> dwarves.h | 68 +++--
> dwarves_fprintf.c | 13 +
> 6 files changed, 693 insertions(+), 180 deletions(-)
>
> --
> 2.39.1
>
--
- Arnaldo
