Re: [PATCH dwarves v2 0/9] pahole: Encode true signatures in kernel BTF

On 09/03/2026 15:32, Yonghong Song wrote:
> 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 17 functions
> cannot have true signatures due to locations.
>

hi Yonghong, one high-level question before I start digging into this further.
Are there any minimum requirements on LLVM/clang version for this support? Thanks!

Alan
 
> For arm64, there are 863 kernel functions having signature changed, and
> 79 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 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.
> 
> Changelog:
>   v1 -> v2:
>     - v1: https://lore.kernel.org/bpf/[email protected]/
>     - 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 (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                                 |  13 +-
>  dwarf_loader.c                                | 397 +++++++++++++++++-
>  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, 662 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%)
>