Re: [PATCH] genksyms: Fix parsing a declarator with a preceding attribute

[Nicolas Schier <nsc@kernel.org>]

On Wed, Feb 25, 2026 at 03:07:17PM -0700, Nathan Chancellor wrote:
> After commit 07919126ecfc ("netfilter: annotate NAT helper hook pointers
> with __rcu"), genksyms fails to parse the __rcu annotation when building
> with CONFIG_DEBUG_INFO_BTF=y, CONFIG_PAHOLE_HAS_BTF_TAG=y, and a version
> of clang that supports btf_type_tag.
> 
>   $ clang --version | head -1
>   ClangBuiltLinux clang version 22.1.0 (https://github.com/llvm/llvm-project.git 4434dabb69916856b824f68a64b029c67175e532)
> 
>   $ cat kernel/configs/repro.config
>   CONFIG_BPF_SYSCALL=y
>   CONFIG_MODVERSIONS=y
>   # CONFIG_DEBUG_INFO_NONE is not set
>   CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
>   CONFIG_DEBUG_INFO_BTF=y
> 
>   $ make -skj"$(nproc)" ARCH=x86_64 LLVM=1 mrproper defconfig repro.config all
>   WARNING: modpost: EXPORT symbol "nf_nat_ftp_hook" [vmlinux] version generation failed, symbol will not be versioned.
>   ...
>   WARNING: modpost: EXPORT symbol "nf_nat_irc_hook" [vmlinux] version generation failed, symbol will not be versioned.
>   ...
> 
> genksyms falls over parsing the __rcu attribute in the declarator:
> 
>   # Kernel reproducer
>   $ make -skj"$(nproc)" ARCH=x86_64 KCFLAGS=-D__GENKSYMS__ LLVM=1 net/netfilter/nf_conntrack_ftp.i
> 
>   $ scripts/genksyms/genksyms -w <net/netfilter/nf_conntrack_ftp.i &| rg 'syntax error'
>   include/linux/netfilter/nf_conntrack_ftp.h:29: syntax error
>   net/netfilter/nf_conntrack_ftp.c:46: syntax error
> 
>   # Trivial reproducer
>   $ cat test.c
>   int (*func)(void *foo, int bar);
>   int (__attribute__((btf_type_tag("rcu"))) *func_with_attr)(void *foo, int bar);
> 
>   $ scripts/genksyms/genksyms -w <test.c
>   <stdin>:2: syntax error
> 
> Optionally allow an attribute to precede a declarator to resolve this
> error and properly generate symbol versions.
> 
> Fixes: 07919126ecfc ("netfilter: annotate NAT helper hook pointers with __rcu")
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
> I plan to take this via the Kbuild tree for 7.0.
> ---
>  scripts/genksyms/parse.y | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 

Thanks for the detailed explanation and reproducer!

Tested-by: Nicolas Schier <nsc@kernel.org>
Reviewed-by: Nicolas Schier <nsc@kernel.org>