[RFC PATCH v1 00/10] bpf: Introduce file dynptr

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

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.

Open challenges:
Patch 6 introduces mechanism for handling dynptrs destruction in
kfuncs, it feels hacky as we need to check for concrete kfunc id to tell
verifier to destroy dynptr, also the way we pass release_regno could be
better. I'm open to suggestions on how to implement this.

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 (10):
  selftests/bpf: remove unnecessary kfunc prototypes
  bpf: widen dynptr size/offset to 64 bit
  lib: extract freader into a separate files
  lib/freader: support reading more than 2 folios
  bpf: verifier: centralize const dynptr check in
    unmark_stack_slots_dynptr()
  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

 include/linux/bpf.h                           |  30 ++-
 include/linux/bpf_verifier.h                  |   2 +
 include/linux/freader.h                       |  32 +++
 kernel/bpf/helpers.c                          | 170 +++++++++---
 kernel/bpf/log.c                              |   2 +
 kernel/bpf/verifier.c                         | 137 ++++++----
 kernel/trace/bpf_trace.c                      |  46 ++--
 lib/Makefile                                  |   2 +-
 lib/buildid.c                                 | 145 +----------
 lib/freader.c                                 | 138 ++++++++++
 tools/testing/selftests/bpf/bpf_kfuncs.h      |  12 +-
 .../selftests/bpf/prog_tests/file_reader.c    |  81 ++++++
 .../selftests/bpf/progs/dynptr_success.c      |  12 +-
 .../testing/selftests/bpf/progs/file_reader.c | 241 ++++++++++++++++++
 .../selftests/bpf/progs/file_reader_fail.c    |  57 +++++
 .../selftests/bpf/progs/ip_check_defrag.c     |   5 -
 .../bpf/progs/verifier_netfilter_ctx.c        |   5 -
 17 files changed, 840 insertions(+), 277 deletions(-)
 create mode 100644 include/linux/freader.h
 create mode 100644 lib/freader.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/file_reader.c
 create mode 100644 tools/testing/selftests/bpf/progs/file_reader.c
 create mode 100644 tools/testing/selftests/bpf/progs/file_reader_fail.c

-- 
2.51.0