Re: [PATCH] perf build: Build error in libbpf missing initialization

[Jakub Kicinski]

On Fri, 27 Jul 2018 10:21:26 +0200, Thomas Richter wrote:
> In linux-next tree compiling the perf tool with additional make flags
> "EXTRA_CFLAGS="-Wp,-D_FORTIFY_SOURCE=2 -O2"
> causes a compiler error. It is the warning
> 	'variable may be used uninitialized'
> which is treated as error:
> 
> I compile it using a FEDORA 28 installation, my gcc compiler version:
> gcc (GCC) 8.0.1 20180324 (Red Hat 8.0.1-0.20)
> 
> The file that causes the error is tools/lib/bpf/libbpf.c
> 
> Here is the error message:
> 
> [root@p23lp27] # make V=1 EXTRA_CFLAGS="-Wp,-D_FORTIFY_SOURCE=2 -O2"
> [...]
> Makefile.config:849: No openjdk development package found, please
>    install JDK package, e.g. openjdk-8-jdk, java-1.8.0-openjdk-devel
> Warning: Kernel ABI header at 'tools/include/uapi/linux/if_link.h'
>         differs from latest version at 'include/uapi/linux/if_link.h'
>   CC       libbpf.o
> libbpf.c: In function âbpf_perf_event_read_simpleâ:
> libbpf.c:2342:6: error: âretâ may be used uninitialized in this
> 			function [-Werror=maybe-uninitialized]
>   int ret;
>       ^
> cc1: all warnings being treated as errors
> mv: cannot stat './.libbpf.o.tmp': No such file or directory
> /home6/tmricht/linux-next/tools/build/Makefile.build:96: recipe for target 'libbpf.o' failed
> 
> Fix this warning and add an addition check at the beginning
> of the while loop.
> 
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> 
> Suggested-by: Jakub Kicinski <jakub.kicinski@xxxxxxxxxxxxx>
> Signed-off-by: Thomas Richter <tmricht@xxxxxxxxxxxxx>

Ah, you already sent this, LGTM, thanks Thomas!

>  tools/lib/bpf/libbpf.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 73465caa33ba..66965ca96113 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -2349,6 +2349,8 @@ bpf_perf_event_read_simple(void *mem, unsigned long size,
>  
>  	begin = base + data_tail % size;
>  	end = base + data_head % size;
> +	if (begin == end)
> +		return LIBBPF_PERF_EVENT_ERROR;
>  
>  	while (begin != end) {
>  		struct perf_event_header *ehdr;