Re: [PATCH v2 4/6] tools/build: Allow versioning LLVM readelf

On 5/18/26 2:03 AM, James Clark wrote:
> Documentation/kbuild/llvm.rst mentions that readelf is included in the
> LLVM toolchain, but it's not currently included in this block.
> 
> Add it so that LLVM=... options also apply to readelf. Users in tools/
> were Perf which was hardcoding it, and another was the BPF makefile.
> Both already include Makefile.include so convert them to use the new
> variable.
> 
> It also didn't have the cross compile prefix, so either readelf didn't
> mind opening cross binaries, or it wasn't working for cross builds.

I'm pretty sure it's the former. readelf/llvm-readelf are only used
libbpf makefile to read ELF symbol tables, which should be arch-independent.

We've been cross-compiling the kernel (and libbpf) on BPF CI for a
long time. So the unprefixed readelf is already working, and adding
the $(CROSS_COMPILE) prefix only adds a new requirement.

I don't think this change in tools/lib/bpf/Makefile is a good idea,
we could potentially break some environments. Even though hardcoded
readelf doesn't look nice.


> 
> Signed-off-by: James Clark <[email protected]>
> ---
>  tools/lib/bpf/Makefile         | 8 ++++----
>  tools/perf/Makefile.perf       | 1 -
>  tools/scripts/Makefile.include | 2 ++
>  3 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
> index 168140f8e646..180dca9c57c8 100644
> --- a/tools/lib/bpf/Makefile
> +++ b/tools/lib/bpf/Makefile
> @@ -114,12 +114,12 @@ PC_FILE		:= $(addprefix $(OUTPUT),$(PC_FILE))
>  
>  TAGS_PROG := $(if $(shell which etags 2>/dev/null),etags,ctags)
>  
> -GLOBAL_SYM_COUNT = $(shell readelf -s --wide $(BPF_IN_SHARED) | \
> +GLOBAL_SYM_COUNT = $(shell $(READELF) -s --wide $(BPF_IN_SHARED) | \
>  			   cut -d "@" -f1 | sed 's/_v[0-9]_[0-9]_[0-9].*//' | \
>  			   sed 's/\[.*\]//' | \
>  			   awk '/GLOBAL/ && /DEFAULT/ && !/UND|ABS/ {print $$NF}' | \
>  			   sort -u | wc -l)
> -VERSIONED_SYM_COUNT = $(shell readelf --dyn-syms --wide $(OUTPUT)libbpf.so | \
> +VERSIONED_SYM_COUNT = $(shell $(READELF) --dyn-syms --wide $(OUTPUT)libbpf.so | \
>  			      sed 's/\[.*\]//' | \
>  			      awk '/GLOBAL/ && /DEFAULT/ && !/UND|ABS/ {print $$NF}' | \
>  			      grep -Eo '[^ ]+@LIBBPF_' | cut -d@ -f1 | sort -u | wc -l)
> @@ -182,12 +182,12 @@ check_abi: $(OUTPUT)libbpf.so $(VERSION_SCRIPT)
>  		     "versioned symbols in $^ ($(VERSIONED_SYM_COUNT))." \
>  		     "Please make sure all LIBBPF_API symbols are"	 \
>  		     "versioned in $(VERSION_SCRIPT)." >&2;		 \
> -		readelf -s --wide $(BPF_IN_SHARED) |			 \
> +		$(READELF) -s --wide $(BPF_IN_SHARED) |			 \
>  		    cut -d "@" -f1 | sed 's/_v[0-9]_[0-9]_[0-9].*//' |	 \
>  		    sed 's/\[.*\]//' |					 \
>  		    awk '/GLOBAL/ && /DEFAULT/ && !/UND/ {print $$NF}'|  \
>  		    sort -u > $(OUTPUT)libbpf_global_syms.tmp;		 \
> -		readelf --dyn-syms --wide $(OUTPUT)libbpf.so |		 \
> +		$(READELF) --dyn-syms --wide $(OUTPUT)libbpf.so |		 \

>  		    sed 's/\[.*\]//' |					 \
>  		    awk '/GLOBAL/ && /DEFAULT/ && !/UND|ABS/ {print $$NF}'|  \
>  		    grep -Eo '[^ ]+@LIBBPF_' | cut -d@ -f1 |		 \
> diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
> index 0aba14f22a06..63276bf55856 100644
> --- a/tools/perf/Makefile.perf
> +++ b/tools/perf/Makefile.perf
> @@ -215,7 +215,6 @@ FLEX    ?= flex
>  BISON   ?= bison
>  STRIP   = strip
>  AWK     = awk
> -READELF ?= readelf
>  
>  # include Makefile.config by default and rule out
>  # non-config cases
> diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
> index e81e5b479c56..380ad84ac51e 100644
> --- a/tools/scripts/Makefile.include
> +++ b/tools/scripts/Makefile.include
> @@ -73,6 +73,7 @@ ifneq ($(LLVM),)
>    $(call allow-override,LLC,$(LLVM_PREFIX)llc$(LLVM_SUFFIX))
>    $(call allow-override,LLVM_CONFIG,$(LLVM_PREFIX)llvm-config$(LLVM_SUFFIX))
>    $(call allow-override,LLVM_OBJCOPY,$(LLVM_PREFIX)llvm-objcopy$(LLVM_SUFFIX))
> +  $(call allow-override,READELF,$(LLVM_PREFIX)llvm-readelf$(LLVM_SUFFIX))
>  else
>    # Allow setting various cross-compile vars or setting CROSS_COMPILE as a prefix.
>    $(call allow-override,CC,$(CROSS_COMPILE)gcc)
> @@ -80,6 +81,7 @@ else
>    $(call allow-override,LD,$(CROSS_COMPILE)ld)
>    $(call allow-override,CXX,$(CROSS_COMPILE)g++)
>    $(call allow-override,STRIP,$(CROSS_COMPILE)strip)
> +  $(call allow-override,READELF,$(CROSS_COMPILE)readelf)
>  
>    # Host versions aren't prefixed
>    $(call allow-override,HOSTAR,ar)
>