[PATCH dwarves 0/9] 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 RDEX 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. There are 69103 kernel functions and 875
kernel functions have signature changed. A series of patches are intended to
ensure true signatures are properly represented. Eventually, only 17 functions
cannot have true signatures due to locations.

For the patch set, Patch 1 introduced usage of DW_AT_calling_convention, which
can precisely identify which function has signature changed. This can filter
majority of functions where their signature won't change.
Patches 2 to 7 tried to find functions with true signature.
Patch 8 enables to btf encoder to properly generate BTF.
Patch 9 includes a few tests.

Yonghong Song (9):
  dwarf_loader: Reduce parameter checking with clang
    DW_AT_calling_convention attr
  dwarf_loader: Handle signatures with dead arguments
  dwarf_loader: Refactor initial ret -1 to be macro PARM_DEFAULT_FAIL
  dwarf_laoder: Handle locations with DW_OP_fbreg
  dwarf_loader: Change exprlen checking condition in parameter__reg()
  dwarf_loader: Detect optimized parameters with locations having
    constant values
  dwarf_loader: Handle expression lists
  btf_encoder: Handle optimized parameter properly
  tests: Add a few clang true signature tests

 btf_encoder.c                                 |  11 +-
 dwarf_loader.c                                | 375 +++++++++++++++++-
 dwarves.h                                     |   3 +
 tests/true_signatures/clang_parm_aggregate.sh |  83 ++++
 tests/true_signatures/clang_parm_optimized.sh |  95 +++++
 .../clang_parm_optimized_stack.sh             |  95 +++++
 .../gcc_true_signatures.sh                    |   0
 7 files changed, 638 insertions(+), 24 deletions(-)
 create mode 100755 tests/true_signatures/clang_parm_aggregate.sh
 create mode 100755 tests/true_signatures/clang_parm_optimized.sh
 create mode 100755 tests/true_signatures/clang_parm_optimized_stack.sh
 rename tests/{ => true_signatures}/gcc_true_signatures.sh (100%)

-- 
2.47.3