Merge branch 'bpf-introduce-file-dynptr'

Mykyta Yatsenko says:

====================
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 validates BPF program reads the
same data as userspace, page faults are enabled in sleepable context and
disabled in non-sleepable.

Changelog:
---
v4 -> v5
v4: https://lore.kernel.org/all/20251021200334.220542-1-mykyta.yatsenko5@gmail.com/
 * Inlined and removed kfunc_call_imm(), run overflow check for call_imm
 only if !bpf_jit_supports_far_kfunc_call().

v3 -> v4
v3: https://lore.kernel.org/bpf/20251020222538.932915-1-mykyta.yatsenko5@gmail.com/
 * Remove ringbuf usage from selftests
 * bpf_dynptr_set_null(ptr) when discarding file dynptr
 * call kfunc_call_imm() in specialize_kfunc() only, removed
 call from add_kfunc_call()

v2 -> v3
v2: https://lore.kernel.org/bpf/20251015161155.120148-1-mykyta.yatsenko5@gmail.com/
 * Add negative tests
 * Rewrote tests to use LSM for bpf_get_task_exe_file()
 * Move call_imm overflow check into kfunc_call_imm()

v1 -> v2
v1: https://lore.kernel.org/bpf/20251003160416.585080-1-mykyta.yatsenko5@gmail.com/
 * Remove ELF parsing selftest
 * Expanded u32 -> u64 refactoring, changes in include/uapi/linux/bpf.h
 * Removed freader.{c,h}, instead move freader definitions into
 buildid.h.
 * Small refactoring of the multiple folios reading algorithm
 * Directly return error after unmark_stack_slots_dynptr().
 * Make kfuncs receive trusted arguments.
 * Remove enum bpf_is_sleepable, use bool instead
 * Remove unnecessary sorting from specialize_kfunc()
 * Remove bool kfunc_in_sleepable_ctx; field from the struct
 bpf_insn_aux_data, rely on non_sleepable field introduced by Kumar
 * Refactor selftests, do madvise(...MADV_PAGEOUT) for all pages read by
 the test
 * Introduce the test for non-sleepable case, verify it fails with -EFAULT
====================

Link: https://lore.kernel.org/r/20251026203853.135105-1-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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