[RFC,dwarves,0/1] pahole: Save header guard names when --header_guards_db is passed

Message

Eduard Zingerman Oct. 25, 2022, 10:07 p.m. UTC

This is a supporting change for a kernel RFC that should allow using
uapi headers with vmlinux.h. For this it is necessary to add
`#ifndef GUARD ... #ifdef` brackets around some types if those types
originate from uapi kernel headers.

For example:

include/uapi/linux/tcp.h:

  #ifndef _UAPI_LINUX_TCP_H
  #define _UAPI_LINUX_TCP_H
  ...
  union tcp_word_hdr {
	struct tcphdr hdr;
	__be32        words[5];
  };
  ...
  #endif /* _UAPI_LINUX_TCP_H */

vmlinux.h:

  ...
  #ifndef _UAPI_LINUX_TCP_H

  union tcp_word_hdr {
	struct tcphdr hdr;
	__be32 words[5];
  };

  #endif
  ...

The information about the GUARD names has to be encoded in BTF to
allow `bpftool` to generate it for a specific kernel binary.

This is achieved by adding a parameter `--header_guards_db` to pahole.
This parameter should point to a file that associates header file
names with names of a C pre-processor variables used as a double
include guards (dubbed as "header guard"s).

For each emitted type the DWARF attribute DW_AT_decl_file is checked,
when the file name is present in the header guards DB a fake
BTF_KIND_DECL_TAG record of the following form is emitted:
- type: the id of the emitted type;
- name_off: a string "header_guard:<guard>".

I'd like to cover this thing with a few unit tests but I don't see any
tests in the dwarves repository ([1]), could you please suggest the
right location?

I'm going to use a lore link to this thread in the kernel RFC email.
I will post a link to the kernel RFC as a reply in this thread as soon
as I have one. The change is not useful on it's own, so if kernel RFC
will not be accepted this one should not be accepted either.

Thanks,
Eduard

[1] https://github.com/acmel/dwarves

Eduard Zingerman (1):
  pahole: Save header guard names when --header_guards_db is passed

 btf_encoder.c | 103 ++++++++++++++++++++++++++++++++++++++++++++++----
 btf_encoder.h |   3 +-
 dutil.c       |  20 +++++++---
 dutil.h       |   1 +
 dwarves.h     |   1 +
 pahole.c      |  99 +++++++++++++++++++++++++++++++++++++++++++++++-
 6 files changed, 210 insertions(+), 17 deletions(-)