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

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

[Thomas Richter]

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

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 ‘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>

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;

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

[Daniel Borkmann]

On 07/27/2018 07:59 PM, Jakub Kicinski wrote:
> 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 ‘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>
> 
> 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;

One question though, any objections to go for something like the below instead?
I doubt we ever hit this in a 'normal' situation, and given we already test for
the begin and end anyway, we could just avoid the extra test altogether. I could
change it to the below if you're good as well (no need to resend anything):

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index d881d37..1aafdbe 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2273,8 +2273,8 @@ bpf_perf_event_read_simple(void *mem, unsigned long size,
        volatile struct perf_event_mmap_page *header = mem;
        __u64 data_tail = header->data_tail;
        __u64 data_head = header->data_head;
+       int ret = LIBBPF_PERF_EVENT_ERROR;
        void *base, *begin, *end;
-       int ret;

        asm volatile("" ::: "memory"); /* in real code it should be smp_rmb() */
        if (data_head == data_tail)

Thanks,
Daniel

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

[Jakub Kicinski]

On Fri, 27 Jul 2018 21:31:01 +0200, Daniel Borkmann wrote:
> On 07/27/2018 07:59 PM, Jakub Kicinski wrote:
> > 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 ‘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>  
> > 
> > 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;  
> 
> One question though, any objections to go for something like the below instead?
> I doubt we ever hit this in a 'normal' situation, and given we already test for
> the begin and end anyway, we could just avoid the extra test altogether. I could
> change it to the below if you're good as well (no need to resend anything):
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index d881d37..1aafdbe 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -2273,8 +2273,8 @@ bpf_perf_event_read_simple(void *mem, unsigned long size,
>         volatile struct perf_event_mmap_page *header = mem;
>         __u64 data_tail = header->data_tail;
>         __u64 data_head = header->data_head;
> +       int ret = LIBBPF_PERF_EVENT_ERROR;
>         void *base, *begin, *end;
> -       int ret;
> 
>         asm volatile("" ::: "memory"); /* in real code it should be smp_rmb() */
>         if (data_head == data_tail)

No real objection, although as a matter of personal taste I'm not a big
fan of initializing err/ret variables unless the code is explicitly
structured to make use of it.  Here it looks slightly more like
silencing a compiler warning, hence my preference to address the actual
cause of the warning rather than catch all.  I guess one could argue
the other way, i.e. if the loop never run (and therefore ret was not
overwritten) there must be *some* error.  I like verbose/explicit code I
guess..

Up to you :)

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

[Daniel Borkmann]

On 07/27/2018 09:56 PM, Jakub Kicinski wrote:
> On Fri, 27 Jul 2018 21:31:01 +0200, Daniel Borkmann wrote:
>> On 07/27/2018 07:59 PM, Jakub Kicinski wrote:
>>> 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 ‘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>  
>>>
>>> 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;  
>>
>> One question though, any objections to go for something like the below instead?
>> I doubt we ever hit this in a 'normal' situation, and given we already test for
>> the begin and end anyway, we could just avoid the extra test altogether. I could
>> change it to the below if you're good as well (no need to resend anything):
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index d881d37..1aafdbe 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -2273,8 +2273,8 @@ bpf_perf_event_read_simple(void *mem, unsigned long size,
>>         volatile struct perf_event_mmap_page *header = mem;
>>         __u64 data_tail = header->data_tail;
>>         __u64 data_head = header->data_head;
>> +       int ret = LIBBPF_PERF_EVENT_ERROR;
>>         void *base, *begin, *end;
>> -       int ret;
>>
>>         asm volatile("" ::: "memory"); /* in real code it should be smp_rmb() */
>>         if (data_head == data_tail)
> 
> No real objection, although as a matter of personal taste I'm not a big
> fan of initializing err/ret variables unless the code is explicitly
> structured to make use of it.  Here it looks slightly more like
> silencing a compiler warning, hence my preference to address the actual
> cause of the warning rather than catch all.  I guess one could argue
> the other way, i.e. if the loop never run (and therefore ret was not
> overwritten) there must be *some* error.  I like verbose/explicit code I
> guess..
> 
> Up to you :)

Ok, I pushed this variant out to the bpf tree since it also is affected there.
Thanks a lot everyone!