Re: pahole and gcc-14 issues

[Alan Maguire <alan.maguire@oracle.com>]

On 25/04/2025 15:50, Alexei Starovoitov wrote:
> Hi All,
> 
> Looks like pahole fails to deduplicate BTF when kernel and
> kernel module are built with gcc-14.
> I see this issue with various kernel .config-s on bpf and
> bpf-next trees.
> I tried pahole 1.28 and the latest master. Same issues.
> 
> BTF in bpf_testmod.ko built with gcc-14 has 2849 types.
> When built with gcc-13 it has 454 types.
> So something is confusing dedup logic.
> Would be great if dedup experts can take a look,
> since this dedup issue is breaking a lot of selftests/bpf.
> 
> Also vmlinux.h generated out of the kernel compiled with gcc-13
> and out of the kernel compiled with gcc-14 shows these differences:
> 
> --- vmlinux13.h    2025-04-24 21:33:50.556884372 -0700
> +++ vmlinux14.h    2025-04-24 21:39:10.310488992 -0700
> @@ -148815,7 +148815,6 @@
>  extern int hid_bpf_input_report(struct hid_bpf_ctx *ctx, enum
> hid_report_type type, u8 *buf, const size_t buf__sz) __weak __ksym;
>  extern void hid_bpf_release_context(struct hid_bpf_ctx *ctx) __weak __ksym;
>  extern int hid_bpf_try_input_report(struct hid_bpf_ctx *ctx, enum
> hid_report_type type, u8 *buf, const size_t buf__sz) __weak __ksym;
> -extern bool scx_bpf_consume(u64 dsq_id) __weak __ksym;
>  extern int scx_bpf_cpu_node(s32 cpu) __weak __ksym;
>  extern struct rq *scx_bpf_cpu_rq(s32 cpu) __weak __ksym;
>  extern u32 scx_bpf_cpuperf_cap(s32 cpu) __weak __ksym;
> @@ -148825,12 +148824,8 @@
>  extern void scx_bpf_destroy_dsq(u64 dsq_id) __weak __ksym;
>  extern void scx_bpf_dispatch(struct task_struct *p, u64 dsq_id, u64
> slice, u64 enq_flags) __weak __ksym;
>  extern void scx_bpf_dispatch_cancel(void) __weak __ksym;
> -extern bool scx_bpf_dispatch_from_dsq(struct bpf_iter_scx_dsq
> *it__iter, struct task_struct *p, u64 dsq_id, u64 enq_flags) __weak
> __ksym;
> -extern void scx_bpf_dispatch_from_dsq_set_slice(struct
> bpf_iter_scx_dsq *it__iter, u64 slice) __weak __ksym;
>  extern void scx_bpf_dispatch_from_dsq_set_vtime(struct
> bpf_iter_scx_dsq *it__iter, u64 vtime) __weak __ksym;
>  extern u32 scx_bpf_dispatch_nr_slots(void) __weak __ksym;
> -extern void scx_bpf_dispatch_vtime(struct task_struct *p, u64 dsq_id,
> u64 slice, u64 vtime, u64 enq_flags) __weak __ksym;
> -extern bool scx_bpf_dispatch_vtime_from_dsq(struct bpf_iter_scx_dsq
> *it__iter, struct task_struct *p, u64 dsq_id, u64 enq_flags) __weak
> __ksym;
>  extern void scx_bpf_dsq_insert(struct task_struct *p, u64 dsq_id, u64
> slice, u64 enq_flags) __weak __ksym;
>  extern void scx_bpf_dsq_insert_vtime(struct task_struct *p, u64
> dsq_id, u64 slice, u64 vtime, u64 enq_flags) __weak __ksym;
>  extern bool scx_bpf_dsq_move(struct bpf_iter_scx_dsq *it__iter,
> struct task_struct *p, u64 dsq_id, u64 enq_flags) __weak __ksym;
> 
> gcc-14's kernel is clearly wrong.
> These 5 kfuncs still exist in the kernel.
> I manually checked there is no if __GNUC__ > 13 in the code.
> Also:
> nm bld/vmlinux|grep -w scx_bpf_consume
> ffffffff8159d4b0 T scx_bpf_consume
> ffffffff8120ea81 t scx_bpf_consume.cold
> 
> I suspect the second issue is not related to the dedup problem.
> All 5 missing kfuncs have ".cold" optimized bodies.
> But ".cold" maybe a red herring, since
> nm bld/vmlinux|grep -w scx_bpf_dispatch
> ffffffff8159d020 T scx_bpf_dispatch
> ffffffff8120ea0f t scx_bpf_dispatch.cold
> but this kfunc is present in vmlinux14.h
> 
> If it makes a difference I have these configs:
> # CONFIG_DEBUG_INFO_DWARF4 is not set
> # CONFIG_DEBUG_INFO_DWARF5 is not set
> # CONFIG_DEBUG_INFO_REDUCED is not set
> CONFIG_DEBUG_INFO_COMPRESSED_NONE=y
> # CONFIG_DEBUG_INFO_COMPRESSED_ZLIB is not set
> # CONFIG_DEBUG_INFO_SPLIT is not set
> CONFIG_DEBUG_INFO_BTF=y
> CONFIG_PAHOLE_HAS_SPLIT_BTF=y
> CONFIG_DEBUG_INFO_BTF_MODULES=y

thanks for the report! I've just reproduced this now with gcc 14; my
initial theory was it might be DWARF5-related, but dedup issues occur
for modules with CONFIG_DEBUG_INFO_DWARF4=y also. I'm seeing task_struct
duplicates in module BTF among other things, so I will try and dig
further and report back when I find something. Like you I suspect the
issues with missing kfuncs are different; may be an issue with our logic
handling inconsistent functions getting confused by the .cold
components. But right now understanding dedup issues is the top priority.

Alan