[PATCH bpf v2] resolve_btfids: Fix linker flags detection

[PATCH bpf v2] resolve_btfids: Fix linker flags detection

[Ihor Solodrai]

The "|| echo -lzstd" default makes zstd an unconditional link
dependency of resolve_btfids. On systems where libzstd-dev is not
installed and pkg-config fails, the linker fails:

  ld: cannot find -lzstd: No such file or directory

libzstd is a transitive dependency of libelf, so the -lzstd flag is
strictly necessary only for static builds [1].

Remove ZSTD_LIBS variable, and instead set LIBELF_LIBS depending on
whether the build is static or not. Use $(HOSTPKG_CONFIG) as primary
source of the flags list.

Also add a default value for HOSTPKG_CONFIG in case it's not built via
the toplevel Makefile.

[1] https://lore.kernel.org/bpf/4ff82800-2daa-4b9f-95a9-6f512859ee70@linux.dev/

Reported-by: BPF CI Bot (Claude Opus 4.6) <bot+bpf-ci@kernel.org>
Reported-by: Vitaly Chikunov <vt@altlinux.org>
Closes: https://lore.kernel.org/bpf/aaWqMcK-2AQw5dx8@altlinux.org/
Fixes: 4021848a903e ("selftests/bpf: Pass through build flags to bpftool and resolve_btfids")
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>

---

v1: https://lore.kernel.org/bpf/20260302231058.916946-1-ihor.solodrai@linux.dev/

---
 tools/bpf/resolve_btfids/Makefile | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/tools/bpf/resolve_btfids/Makefile b/tools/bpf/resolve_btfids/Makefile
index ef083602b73a..7672208f65e4 100644
--- a/tools/bpf/resolve_btfids/Makefile
+++ b/tools/bpf/resolve_btfids/Makefile
@@ -23,6 +23,7 @@ RM      ?= rm
 HOSTCC  ?= gcc
 HOSTLD  ?= ld
 HOSTAR  ?= ar
+HOSTPKG_CONFIG ?= pkg-config
 CROSS_COMPILE =
 
 OUTPUT ?= $(srctree)/tools/bpf/resolve_btfids/
@@ -63,10 +64,14 @@ $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OU
 		    $(abspath $@) install_headers
 
 LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
+
+ifneq ($(filter -static,$(EXTRA_LDFLAGS)),)
+LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs --static 2>/dev/null || echo -lelf -lzstd)
+else
 LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
+endif
 
 ZLIB_LIBS  := $(shell $(HOSTPKG_CONFIG) zlib --libs 2>/dev/null || echo -lz)
-ZSTD_LIBS  := $(shell $(HOSTPKG_CONFIG) libzstd --libs 2>/dev/null || echo -lzstd)
 
 HOSTCFLAGS_resolve_btfids += -g \
           -I$(srctree)/tools/include \
@@ -76,7 +81,7 @@ HOSTCFLAGS_resolve_btfids += -g \
           $(LIBELF_FLAGS) \
           -Wall -Werror
 
-LIBS = $(LIBELF_LIBS) $(ZLIB_LIBS) $(ZSTD_LIBS)
+LIBS = $(LIBELF_LIBS) $(ZLIB_LIBS)
 
 export srctree OUTPUT HOSTCFLAGS_resolve_btfids Q HOSTCC HOSTLD HOSTAR
 include $(srctree)/tools/build/Makefile.include
-- 
2.53.0

Re: [PATCH bpf v2] resolve_btfids: Fix linker flags detection

[Vitaly Chikunov]

Ihor,

On Tue, Mar 03, 2026 at 11:39:54AM -0800, Ihor Solodrai wrote:
> The "|| echo -lzstd" default makes zstd an unconditional link
> dependency of resolve_btfids. On systems where libzstd-dev is not
> installed and pkg-config fails, the linker fails:
> 
>   ld: cannot find -lzstd: No such file or directory
> 
> libzstd is a transitive dependency of libelf, so the -lzstd flag is
> strictly necessary only for static builds [1].
> 
> Remove ZSTD_LIBS variable, and instead set LIBELF_LIBS depending on
> whether the build is static or not. Use $(HOSTPKG_CONFIG) as primary
> source of the flags list.
> 
> Also add a default value for HOSTPKG_CONFIG in case it's not built via
> the toplevel Makefile.
> 
> [1] https://lore.kernel.org/bpf/4ff82800-2daa-4b9f-95a9-6f512859ee70@linux.dev/
> 
> Reported-by: BPF CI Bot (Claude Opus 4.6) <bot+bpf-ci@kernel.org>
> Reported-by: Vitaly Chikunov <vt@altlinux.org>
> Closes: https://lore.kernel.org/bpf/aaWqMcK-2AQw5dx8@altlinux.org/
> Fixes: 4021848a903e ("selftests/bpf: Pass through build flags to bpftool and resolve_btfids")
> Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
> 
> ---
> 
> v1: https://lore.kernel.org/bpf/20260302231058.916946-1-ihor.solodrai@linux.dev/
> 
> ---
>  tools/bpf/resolve_btfids/Makefile | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/bpf/resolve_btfids/Makefile b/tools/bpf/resolve_btfids/Makefile
> index ef083602b73a..7672208f65e4 100644
> --- a/tools/bpf/resolve_btfids/Makefile
> +++ b/tools/bpf/resolve_btfids/Makefile
> @@ -23,6 +23,7 @@ RM      ?= rm
>  HOSTCC  ?= gcc
>  HOSTLD  ?= ld
>  HOSTAR  ?= ar
> +HOSTPKG_CONFIG ?= pkg-config
>  CROSS_COMPILE =
>  
>  OUTPUT ?= $(srctree)/tools/bpf/resolve_btfids/
> @@ -63,10 +64,14 @@ $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OU
>  		    $(abspath $@) install_headers
>  
>  LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
> +
> +ifneq ($(filter -static,$(EXTRA_LDFLAGS)),)
> +LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs --static 2>/dev/null || echo -lelf -lzstd)
> +else
>  LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
> +endif
>  
>  ZLIB_LIBS  := $(shell $(HOSTPKG_CONFIG) zlib --libs 2>/dev/null || echo -lz)

Perhaps, this line is not needed either, as `pkg-config libelf --libs
--static` will add -lz.

Thanks,

> -ZSTD_LIBS  := $(shell $(HOSTPKG_CONFIG) libzstd --libs 2>/dev/null || echo -lzstd)
>  
>  HOSTCFLAGS_resolve_btfids += -g \
>            -I$(srctree)/tools/include \
> @@ -76,7 +81,7 @@ HOSTCFLAGS_resolve_btfids += -g \
>            $(LIBELF_FLAGS) \
>            -Wall -Werror
>  
> -LIBS = $(LIBELF_LIBS) $(ZLIB_LIBS) $(ZSTD_LIBS)
> +LIBS = $(LIBELF_LIBS) $(ZLIB_LIBS)
>  
>  export srctree OUTPUT HOSTCFLAGS_resolve_btfids Q HOSTCC HOSTLD HOSTAR
>  include $(srctree)/tools/build/Makefile.include
> -- 
> 2.53.0

Re: [PATCH bpf v2] resolve_btfids: Fix linker flags detection

[Ihor Solodrai]

On 3/3/26 12:06 PM, Vitaly Chikunov wrote:
> Ihor,
> 
> [...]
>>  
>>  LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
>> +
>> +ifneq ($(filter -static,$(EXTRA_LDFLAGS)),)
>> +LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs --static 2>/dev/null || echo -lelf -lzstd)
>> +else
>>  LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
>> +endif
>>  
>>  ZLIB_LIBS  := $(shell $(HOSTPKG_CONFIG) zlib --libs 2>/dev/null || echo -lz)
> 
> Perhaps, this line is not needed either, as `pkg-config libelf --libs
> --static` will add -lz.

The -lz has been there forever too [1], and a duplicate lib flag is
harmless here.  We also want to have reasonable defaults if pkg-config
is not available.

That said, the same can be achieved with the diff pasted below.

I don't mind either way, let's see if maintainers have an opinion.

diff --git a/tools/bpf/resolve_btfids/Makefile b/tools/bpf/resolve_btfids/Makefile
index 7672208f65e4..a7dac1587344 100644
--- a/tools/bpf/resolve_btfids/Makefile
+++ b/tools/bpf/resolve_btfids/Makefile
@@ -66,13 +66,11 @@ $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OU
 LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
 
 ifneq ($(filter -static,$(EXTRA_LDFLAGS)),)
-LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs --static 2>/dev/null || echo -lelf -lzstd)
+LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs --static 2>/dev/null || echo -lelf -lzstd -lz)
 else
-LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
+LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf -lz)
 endif
 
-ZLIB_LIBS  := $(shell $(HOSTPKG_CONFIG) zlib --libs 2>/dev/null || echo -lz)
-
 HOSTCFLAGS_resolve_btfids += -g \
           -I$(srctree)/tools/include \
           -I$(srctree)/tools/include/uapi \
@@ -81,7 +79,7 @@ HOSTCFLAGS_resolve_btfids += -g \
           $(LIBELF_FLAGS) \
           -Wall -Werror
 
-LIBS = $(LIBELF_LIBS) $(ZLIB_LIBS)
+LIBS = $(LIBELF_LIBS)
 
 export srctree OUTPUT HOSTCFLAGS_resolve_btfids Q HOSTCC HOSTLD HOSTAR
 include $(srctree)/tools/build/Makefile.include


[1] https://lore.kernel.org/bpf/20200711215329.41165-2-jolsa@kernel.org/

> 
> Thanks,
> 
>> -ZSTD_LIBS  := $(shell $(HOSTPKG_CONFIG) libzstd --libs 2>/dev/null || echo -lzstd)
>>  
>>  HOSTCFLAGS_resolve_btfids += -g \
>>            -I$(srctree)/tools/include \
>> @@ -76,7 +81,7 @@ HOSTCFLAGS_resolve_btfids += -g \
>>            $(LIBELF_FLAGS) \
>>            -Wall -Werror
>>  
>> -LIBS = $(LIBELF_LIBS) $(ZLIB_LIBS) $(ZSTD_LIBS)
>> +LIBS = $(LIBELF_LIBS) $(ZLIB_LIBS)
>>  
>>  export srctree OUTPUT HOSTCFLAGS_resolve_btfids Q HOSTCC HOSTLD HOSTAR
>>  include $(srctree)/tools/build/Makefile.include
>> -- 
>> 2.53.0

Re: [PATCH bpf v2] resolve_btfids: Fix linker flags detection

[Vitaly Chikunov]

Ihor,

On Tue, Mar 03, 2026 at 11:39:54AM -0800, Ihor Solodrai wrote:
> The "|| echo -lzstd" default makes zstd an unconditional link
> dependency of resolve_btfids. On systems where libzstd-dev is not
> installed and pkg-config fails, the linker fails:
> 
>   ld: cannot find -lzstd: No such file or directory
> 
> libzstd is a transitive dependency of libelf, so the -lzstd flag is
> strictly necessary only for static builds [1].
> 
> Remove ZSTD_LIBS variable, and instead set LIBELF_LIBS depending on
> whether the build is static or not. Use $(HOSTPKG_CONFIG) as primary
> source of the flags list.
> 
> Also add a default value for HOSTPKG_CONFIG in case it's not built via
> the toplevel Makefile.
> 
> [1] https://lore.kernel.org/bpf/4ff82800-2daa-4b9f-95a9-6f512859ee70@linux.dev/
> 
> Reported-by: BPF CI Bot (Claude Opus 4.6) <bot+bpf-ci@kernel.org>
> Reported-by: Vitaly Chikunov <vt@altlinux.org>
> Closes: https://lore.kernel.org/bpf/aaWqMcK-2AQw5dx8@altlinux.org/
> Fixes: 4021848a903e ("selftests/bpf: Pass through build flags to bpftool and resolve_btfids")
> Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
> 
> ---
> 
> v1: https://lore.kernel.org/bpf/20260302231058.916946-1-ihor.solodrai@linux.dev/
> 
> ---
>  tools/bpf/resolve_btfids/Makefile | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/bpf/resolve_btfids/Makefile b/tools/bpf/resolve_btfids/Makefile
> index ef083602b73a..7672208f65e4 100644
> --- a/tools/bpf/resolve_btfids/Makefile
> +++ b/tools/bpf/resolve_btfids/Makefile
> @@ -23,6 +23,7 @@ RM      ?= rm
>  HOSTCC  ?= gcc
>  HOSTLD  ?= ld
>  HOSTAR  ?= ar
> +HOSTPKG_CONFIG ?= pkg-config
>  CROSS_COMPILE =
>  
>  OUTPUT ?= $(srctree)/tools/bpf/resolve_btfids/
> @@ -63,10 +64,14 @@ $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OU
>  		    $(abspath $@) install_headers
>  
>  LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
> +
> +ifneq ($(filter -static,$(EXTRA_LDFLAGS)),)
> +LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs --static 2>/dev/null || echo -lelf -lzstd)
> +else
>  LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
> +endif

Btw, pkg-config accepts -static, so you can do just

-LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
+LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs $(filter -static,$(EXTRA_LDFLAGS)) 2>/dev/null || echo -lelf)

Thanks,

>  
>  ZLIB_LIBS  := $(shell $(HOSTPKG_CONFIG) zlib --libs 2>/dev/null || echo -lz)
> -ZSTD_LIBS  := $(shell $(HOSTPKG_CONFIG) libzstd --libs 2>/dev/null || echo -lzstd)
>  
>  HOSTCFLAGS_resolve_btfids += -g \
>            -I$(srctree)/tools/include \
> @@ -76,7 +81,7 @@ HOSTCFLAGS_resolve_btfids += -g \
>            $(LIBELF_FLAGS) \
>            -Wall -Werror
>  
> -LIBS = $(LIBELF_LIBS) $(ZLIB_LIBS) $(ZSTD_LIBS)
> +LIBS = $(LIBELF_LIBS) $(ZLIB_LIBS)
>  
>  export srctree OUTPUT HOSTCFLAGS_resolve_btfids Q HOSTCC HOSTLD HOSTAR
>  include $(srctree)/tools/build/Makefile.include
> -- 
> 2.53.0