bpf: Introduce file dynptr

From: Mykyta Yatsenko <yatsenko@meta.com>

This series adds a new dynptr kind, file dynptr, which enables BPF
programs to perform safe reads from files in a structured way.
Initial motivations include:
 * Parsing the executable’s ELF to locate thread-local variable symbols
 * Capturing stack traces when frame pointers are disabled

By leveraging the existing dynptr abstraction, we reuse the verifier’s
lifetime/size checks and keep the API consistent with existing dynptr
read helpers.

Technical details:
1. Reuses the existing freader library to read files a folio at a time.
2. bpf_dynptr_slice() and bpf_dynptr_read() always copy data from folios
into a program-provided buffer; zero-copy access is intentionally not
supported to keep it simple.
3. Reads may sleep if the requested folios are not in the page cache.
4. Few verifier changes required:
  * Support dynptr destruction in kfuncs
  * Add kfunc address substitution based on whether the program runs in
  a sleepable or non-sleepable context.

Testing:
The final patch adds a selftest that parses the executable’s ELF to
locate thread-local symbol information, demonstrating the file dynptr
workflow end-to-end.

Mykyta Yatsenko (11):
  selftests/bpf: remove unnecessary kfunc prototypes
  bpf: widen dynptr size/offset to 64 bit
  lib: move freader into buildid.h
  lib/freader: support reading more than 2 folios
  bpf: verifier: centralize const dynptr check in
    unmark_stack_slots_dynptr()
  bpf: mark vm_area_struct as trusted
  bpf: add plumbing for file-backed dynptr
  bpf: add kfuncs and helpers support for file dynptrs
  bpf: verifier: refactor kfunc specialization
  bpf: dispatch to sleepable file dynptr
  selftests/bpf: add file dynptr tests

 MAINTAINERS                                   |   1 +
 include/linux/bpf.h                           |  30 +--
 include/linux/buildid.h                       |  25 +++
 include/uapi/linux/bpf.h                      |   8 +-
 kernel/bpf/helpers.c                          | 172 ++++++++++++++----
 kernel/bpf/log.c                              |   2 +
 kernel/bpf/verifier.c                         | 158 ++++++++++------
 kernel/trace/bpf_trace.c                      |  46 ++---
 lib/buildid.c                                 |  55 ++----
 tools/include/uapi/linux/bpf.h                |   8 +-
 tools/testing/selftests/bpf/bpf_kfuncs.h      |  12 +-
 .../selftests/bpf/prog_tests/file_reader.c    | 114 ++++++++++++
 .../selftests/bpf/progs/dynptr_success.c      |  12 +-
 .../testing/selftests/bpf/progs/file_reader.c | 157 ++++++++++++++++
 tools/testing/selftests/bpf/progs/find_vma.c  |   6 +-
 .../selftests/bpf/progs/ip_check_defrag.c     |   5 -
 .../bpf/progs/verifier_netfilter_ctx.c        |   5 -
 17 files changed, 624 insertions(+), 192 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/file_reader.c
 create mode 100644 tools/testing/selftests/bpf/progs/file_reader.c

-- 
2.51.0