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

[Thomas Richter <tmricht@linux.ibm.com>]

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 ‘bpf_perf_event_read_simple’:
libbpf.c:2342:6: error: ‘ret’ 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@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>

Suggested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
---
 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;
-- 
2.16.4