Re: [PATCH dwarves v3 1/9] dwarf_loader: Reduce parameter checking with clang DW_AT_calling_convention attr

Jiri Olsa
2026-03-21T23:10:22Z

On Fri, Mar 20, 2026 at 12:09:22PM -0700, Yonghong Song wrote:
> Currently every function is checked for its parameters to identify whether
> the signature changed or not. If signature indeed changed, pahole may do
> some adjustment for parameters for true signatures.
>
> In clang, any function with the following attribute
> DW_AT_calling_convention (DW_CC_nocall)
> indicates this function having signature changed.
> pahole can take advantage of this to avoid parameter checking if
> DW_AT_calling_convention is not DW_CC_nocall.
>
> But more importantly, DW_CC_nocall can identify signature-changed functions
> and parameters can be checked one-after-another to create the true
> signatures. Otherwise, it takes more effort to identify whether a
> function has signature changed or not. For example, for funciton
> __bpf_kfunc static void bbr_main(struct sock *sk, u32 ack, int flag,
> const struct rate_sample *rs) { ... }
> and bbr_main() is a callback function in
> .cong_control = bbr_main
> in 'struct tcp_congestion_ops tcp_bbr_cong_ops'.
> In the above bbr_main(...), parameter 'ack' and 'flag' are not used.
> The following are some details:
>
> 0x0a713b8d: DW_TAG_formal_parameter
> DW_AT_location (indexed (0x28) loclist = 0x0166d452:
> [0xffffffff83e77fd9, 0xffffffff83e78016): DW_OP_reg5 RDI
> ...
> DW_AT_name ("sk")
> DW_AT_type (0x0a6f5b2b "sock *")
> ...
>
> 0x0a713b98: DW_TAG_formal_parameter
> DW_AT_name ("ack")
> DW_AT_type (0x0a6f58fd "u32")
> ...
>
> 0x0a713ba2: DW_TAG_formal_parameter
> DW_AT_name ("flag")
> DW_AT_type (0x0a6f57d1 "int")
> ...
>
> 0x0a713bac: DW_TAG_formal_parameter
> DW_AT_location (indexed (0x29) loclist = 0x0166d4a8:
> [0xffffffff83e77fd9, 0xffffffff83e78016): DW_OP_reg2 RCX
> ...
> DW_AT_name ("rs")
> DW_AT_type (0x0a710da5 "const rate_sample *")
> DW_AT_decl_line (1027)
>
> Some analysis for the above dwarf can conclude that the 'ark' and 'flag'
> may be related to RSI and RDX, considering the last one is RCX. Basically this
> requires all parameters are available to collectively decide whether the
> true signature can be found or not. In such case, DW_CC_nocall can make things
> easier as parameter can be checked one after another.
>
> For a clang built bpf-next kernel, in non-LTO setup, the number of kernel functions
> is 69103 and the number of signature changed functions is 875, based on
> DW_AT_calling_convention (DW_CC_nocall)
> indication.

we don't display these stats, right? would be great and probably easy
to count different flags and aggregate them

>
> Among 875 signature changed functions, after this patch, 495 functions
> can have proper true signatures, mostly due to simple dead argument
> elimination. The number of remaining functions, which cannot get the
> true signature, is 379. They will be addressed in the subsequent commits.

after this change I have more functions added (below) and I checked
few and they seem ok (w/o DW_CC_nocall)

jirka

acpi_ex_do_debug_object
add_bits
amd_pstate_get_mode_string
dma_alloc_from_contiguous
do_set_pmd
dst_destroy
find_cpio_data
find_microcode_in_initrd
ima_write_policy
insn_init
io_tx_ubuf_complete
ip6_protocol_deliver_rcu
kvm_tdp_mmu_unmap_gfn_range
net_dim_get_def_rx_moderation
net_dim_get_def_tx_moderation
pinctrl_commit_state
string_unescape
syscall_copy_user_array
tdp_mmu_zap_leafs
usb_speed_string
__vlan_find_dev_deep_rcu
__vxlan_fdb_delete
xfs_bmbt_maxrecs
xfs_inobt_maxrecs
ZSTD_buildSequencesStatistics
ZSTD_copyCCtx
ZSTD_findFrameSizeInfo
ZSTD_get1BlockSummary
zstd_get_cparams
ZSTD_getCParams
ZSTD_getCParamsFromCCtxParams
ZSTD_getCParamsFromCDict
ZSTD_getCParams_internal
zstd_get_params
ZSTD_getParams