[PATCH dwarves v6 0/5] pahole: Encode true signatures in kernel BTF

[Yonghong Song <yonghong.song@linux.dev>]

Current vmlinux BTF encoding is based on the source level signatures.
But the compiler may do some optimization and changed the signature.
If the user tried with source level signature, their initial implementation
may have wrong results and then the user need to check what is the
problem and work around it, e.g. through kprobe since kprobe does not
need vmlinux BTF.

Majority of changed signatures are due to dead argument elimination.
The following is a more complex one. The original source signature:
  typedef struct {
        union {
                void            *kernel;
                void __user     *user;
        };
        bool            is_kernel : 1;
  } sockptr_t;
  typedef sockptr_t bpfptr_t;
  static int map_create(union bpf_attr *attr, bpfptr_t uattr) { ... }
After compiler optimization, the signature becomes:
  static int map_create(union bpf_attr *attr, bool uattr__is_kernel) { ... }
In the above, uattr__is_kernel corresponds to 'is_kernel' field in sockptr_t.
This makes it easier for developers to understand what changed.

The new signature needs to properly follow ABI specification based on
locations. Otherwise, that signature should be discarded. For example,

    0x0242f1f7:   DW_TAG_subprogram
                    DW_AT_name      ("memblock_find_in_range")
                    DW_AT_calling_convention        (DW_CC_nocall)
                    DW_AT_type      (0x0242decc "phys_addr_t")
                    ...
    0x0242f22e:     DW_TAG_formal_parameter
                      DW_AT_location        (indexed (0x14a) loclist = 0x005595bc:
                         [0xffffffff87a000f9, 0xffffffff87a00178): DW_OP_reg5 RDI
                         [0xffffffff87a00178, 0xffffffff87a001be): DW_OP_reg14 R14
                         [0xffffffff87a001be, 0xffffffff87a001c7): DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
                         [0xffffffff87a001c7, 0xffffffff87a00214): DW_OP_reg14 R14)
                      DW_AT_name    ("start")
                      DW_AT_type    (0x0242decc "phys_addr_t")
                      ...
    0x0242f239:     DW_TAG_formal_parameter
                      DW_AT_location        (indexed (0x14b) loclist = 0x005595e6:
                         [0xffffffff87a000f9, 0xffffffff87a00175): DW_OP_reg4 RSI
                         [0xffffffff87a00175, 0xffffffff87a001b8): DW_OP_reg3 RBX
                         [0xffffffff87a001b8, 0xffffffff87a001c7): DW_OP_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value
                         [0xffffffff87a001c7, 0xffffffff87a00214): DW_OP_reg3 RBX)
                      DW_AT_name    ("end")
                      DW_AT_type    (0x0242decc "phys_addr_t")
                      ...
    0x0242f245:     DW_TAG_formal_parameter
                      DW_AT_location        (indexed (0x14c) loclist = 0x00559610:
                         [0xffffffff87a001e3, 0xffffffff87a001ef): DW_OP_breg4 RSI+0)
                      DW_AT_name    ("size")
                      DW_AT_type    (0x0242decc "phys_addr_t")
                      ...
    0x0242f250:     DW_TAG_formal_parameter
                      DW_AT_const_value     (4096)
                      DW_AT_name    ("align")
                      DW_AT_type    (0x0242decc "phys_addr_t")
                      ...

The third argument should correspond to RDX for x86_64. But the location suggests that
the parameter value is stored in the address with 'RSI + 0'. It is not clear whether
the parameter value is stored in RDX or not. So we have to discard this funciton in
vmlinux BTF to avoid incorrect true signatures.

For llvm, any function having
  DW_AT_calling_convention        (DW_CC_nocall)
in dwarf DW_TAG_subprogram will indicate that this function has signature changed.
I did experiment with latest bpf-next. For x86_64, there are 69103 kernel functions
and 875 kernel functions having signature changed. A series of patches are intended
to ensure true signatures are properly represented. Eventually, only 20 functions
cannot have true signatures due to locations.

For arm64, there are 863 kernel functions having signature changed, and
108 functions cannot have true signatures due to locations. I checked those
functions and look like llvm arm64 backend more relaxed to compute parameter
values.

For full testing, I enabled true signature support in kernel scripts/Makefile.btf like below:
  -pahole-flags-$(call test-ge, $(pahole-ver), 131) += --btf_features=attributes
  +pahole-flags-$(call test-ge, $(pahole-ver), 131) += --btf_features=attributes --btf_features=+true_signature

See individual patches for details.

Changelog:
  v5 -> v6:
    - v5: https://lore.kernel.org/bpf/20260523165712.1225231-1-yonghong.song@linux.dev/
    - The previous change relies on parameter__new() to collect and analyze each
      parameter to decide true signatures. The new one separates collecting and
      analyzing phase from Alan. This two-phase makes logic easy to understand.
    - In btf_encoder.c, remove usage of skip_idx to simplify the code.
  v4 -> v5:
    - v4: https://lore.kernel.org/bpf/20260326013144.2901265-1-yonghong.song@linux.dev/
    - Check info.signature_changed only under clang.
    - Fix an uninitialized varable issue (var reg_dix) for gcc.
  v3 -> v4:
    - v3: https://lore.kernel.org/bpf/20260320190917.1970524-1-yonghong.song@linux.dev/
    - Add simple prescan of parameter registers in order to get true signatures
      for those functions where optimization could happen but compiler didn't do it.
    - Do not create a new name (e.g. "uattr__is_kernel") with malloc at parameter_reg()
      stage. Instead remember both "uattr" and "is_kernel" and later generate the
      name "uattr_is_kernel" in btf encoder.
    - Add comments to explain how to handle parameters which may take two registers.
    - Fix some test failures on aarch64.
  v2 -> v3:
    - v2: https://lore.kernel.org/bpf/20260309153215.1917033-1-yonghong.song@linux.dev/
    - Change tests by using newly added test_lib.sh.
    - Simplify to get bool variable producer_clang.
    - Try to avoid producer_clang appearance in dwarf_loader.c in order to avoid
      clear separation between clang and gcc.
  v1 -> v2:
    - v1: https://lore.kernel.org/bpf/20260305225455.1151066-1-yonghong.song@linux.dev/
    - Added producer_clang guarding in btf_encoder. Otherwise, gcc kernel build
      will crash pahole.
    - Fix an early return in parameter__reg() which didn't do pthread_mutex_unlock()
      which caused the deadlock for arm64.
    - Add a few more places to guard with producer_clang and conf->true_signature
      to maintain the previous behavior if not clang or conf->true_signature is false.

Yonghong Song (5):
  dwarf_loader: Detect aggregate ABI register usage and signature
    changes
  dwarf_loader: Collect per-parameter information
  dwarf_loader: Analyze per-parameter information for true signatures
  btf_encoder: Emit true function signatures
  tests: add BTF true_signature encoding tests

 btf_encoder.c                       |  24 +-
 dwarf_loader.c                      | 548 ++++++++++++++++++++++++----
 dwarves.h                           |  14 +
 tests/clang_parm_aggregate.sh       |  85 +++++
 tests/clang_parm_memory.sh          |  77 ++++
 tests/clang_parm_optimized.sh       |  63 ++++
 tests/clang_parm_optimized_stack.sh |  63 ++++
 7 files changed, 812 insertions(+), 62 deletions(-)
 create mode 100755 tests/clang_parm_aggregate.sh
 create mode 100755 tests/clang_parm_memory.sh
 create mode 100755 tests/clang_parm_optimized.sh
 create mode 100755 tests/clang_parm_optimized_stack.sh

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2.53.0-Meta