[PATCH net-next] selftests: bpf: modify urandom_read and link it non-statically

[PATCH net-next] selftests: bpf: modify urandom_read and link it non-statically

[Ivan Vecera]

After some experiences I found that urandom_read does not need to be
linked statically. When the 'read' syscall call is moved to separate
non-inlined function then bpf_get_stackid() is able to find
the executable in stack trace and extract its build_id from it.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
---
 tools/testing/selftests/bpf/Makefile       |  2 +-
 tools/testing/selftests/bpf/urandom_read.c | 15 +++++++++++----
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 2aed37ea61a4..c33900a8fec0 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -69,7 +69,7 @@ TEST_CUSTOM_PROGS = $(OUTPUT)/urandom_read
 all: $(TEST_CUSTOM_PROGS)
 
 $(OUTPUT)/urandom_read: $(OUTPUT)/%: %.c
-	$(CC) -o $@ -static $< -Wl,--build-id
+	$(CC) -o $@ $< -Wl,--build-id
 
 BPFOBJ := $(OUTPUT)/libbpf.a
 
diff --git a/tools/testing/selftests/bpf/urandom_read.c b/tools/testing/selftests/bpf/urandom_read.c
index 9de8b7cb4e6d..db781052758d 100644
--- a/tools/testing/selftests/bpf/urandom_read.c
+++ b/tools/testing/selftests/bpf/urandom_read.c
@@ -7,11 +7,19 @@
 
 #define BUF_SIZE 256
 
+static __attribute__((noinline))
+void urandom_read(int fd, int count)
+{
+       char buf[BUF_SIZE];
+       int i;
+
+       for (i = 0; i < count; ++i)
+               read(fd, buf, BUF_SIZE);
+}
+
 int main(int argc, char *argv[])
 {
 	int fd = open("/dev/urandom", O_RDONLY);
-	int i;
-	char buf[BUF_SIZE];
 	int count = 4;
 
 	if (fd < 0)
@@ -20,8 +28,7 @@ int main(int argc, char *argv[])
 	if (argc == 2)
 		count = atoi(argv[1]);
 
-	for (i = 0; i < count; ++i)
-		read(fd, buf, BUF_SIZE);
+	urandom_read(fd, count);
 
 	close(fd);
 	return 0;
-- 
2.19.2

Re: [PATCH net-next] selftests: bpf: modify urandom_read and link it non-statically

[Stanislav Fomichev]

On 03/15, Ivan Vecera wrote:
> After some experiences I found that urandom_read does not need to be
> linked statically. When the 'read' syscall call is moved to separate
> non-inlined function then bpf_get_stackid() is able to find
> the executable in stack trace and extract its build_id from it.
But why? Do you have some problems with it being linked statically?

> 
> Signed-off-by: Ivan Vecera <ivecera@redhat.com>
> ---
>  tools/testing/selftests/bpf/Makefile       |  2 +-
>  tools/testing/selftests/bpf/urandom_read.c | 15 +++++++++++----
>  2 files changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index 2aed37ea61a4..c33900a8fec0 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/tools/testing/selftests/bpf/Makefile
> @@ -69,7 +69,7 @@ TEST_CUSTOM_PROGS = $(OUTPUT)/urandom_read
>  all: $(TEST_CUSTOM_PROGS)
>  
>  $(OUTPUT)/urandom_read: $(OUTPUT)/%: %.c
> -	$(CC) -o $@ -static $< -Wl,--build-id
> +	$(CC) -o $@ $< -Wl,--build-id
>  
>  BPFOBJ := $(OUTPUT)/libbpf.a
>  
> diff --git a/tools/testing/selftests/bpf/urandom_read.c b/tools/testing/selftests/bpf/urandom_read.c
> index 9de8b7cb4e6d..db781052758d 100644
> --- a/tools/testing/selftests/bpf/urandom_read.c
> +++ b/tools/testing/selftests/bpf/urandom_read.c
> @@ -7,11 +7,19 @@
>  
>  #define BUF_SIZE 256
>  
> +static __attribute__((noinline))
> +void urandom_read(int fd, int count)
> +{
> +       char buf[BUF_SIZE];
> +       int i;
> +
> +       for (i = 0; i < count; ++i)
> +               read(fd, buf, BUF_SIZE);
> +}
> +
>  int main(int argc, char *argv[])
>  {
>  	int fd = open("/dev/urandom", O_RDONLY);
> -	int i;
> -	char buf[BUF_SIZE];
>  	int count = 4;
>  
>  	if (fd < 0)
> @@ -20,8 +28,7 @@ int main(int argc, char *argv[])
>  	if (argc == 2)
>  		count = atoi(argv[1]);
>  
> -	for (i = 0; i < count; ++i)
> -		read(fd, buf, BUF_SIZE);
> +	urandom_read(fd, count);
>  
>  	close(fd);
>  	return 0;
> -- 
> 2.19.2
> 

Re: [PATCH net-next] selftests: bpf: modify urandom_read and link it non-statically

[Ivan Vecera]

On 15. 03. 19 21:08, Stanislav Fomichev wrote:
> On 03/15, Ivan Vecera wrote:
>> After some experiences I found that urandom_read does not need to be
>> linked statically. When the 'read' syscall call is moved to separate
>> non-inlined function then bpf_get_stackid() is able to find
>> the executable in stack trace and extract its build_id from it.
> But why? Do you have some problems with it being linked statically?
> 
Dependency... you don't need to install static glibc to compile the bpf samples. 
Shared libc is available everytime.

Ivan

Re: [PATCH net-next] selftests: bpf: modify urandom_read and link it non-statically

[Stanislav Fomichev]

On 03/15, Ivan Vecera wrote:
> On 15. 03. 19 21:08, Stanislav Fomichev wrote:
> > On 03/15, Ivan Vecera wrote:
> > > After some experiences I found that urandom_read does not need to be
> > > linked statically. When the 'read' syscall call is moved to separate
> > > non-inlined function then bpf_get_stackid() is able to find
> > > the executable in stack trace and extract its build_id from it.
> > But why? Do you have some problems with it being linked statically?
> > 
> Dependency... you don't need to install static glibc to compile the bpf
> samples. Shared libc is available everytime.
Oh, the distros that do -devel _and_ -static packages :-)

So your patch essentially adds a call, that leaves a trace on the stack
with our build-id. I guess that works as well.

> 
> Ivan

Re: [PATCH net-next] selftests: bpf: modify urandom_read and link it non-statically

[Ivan Vecera]

----- Stanislav Fomichev <sdf@fomichev.me> wrote:
> On 03/15, Ivan Vecera wrote:
> > On 15. 03. 19 21:08, Stanislav Fomichev wrote:
> > > On 03/15, Ivan Vecera wrote:
> > > > After some experiences I found that urandom_read does not need to be
> > > > linked statically. When the 'read' syscall call is moved to separate
> > > > non-inlined function then bpf_get_stackid() is able to find
> > > > the executable in stack trace and extract its build_id from it.
> > > But why? Do you have some problems with it being linked statically?
> > > 
> > Dependency... you don't need to install static glibc to compile the bpf
> > samples. Shared libc is available everytime.
> Oh, the distros that do -devel _and_ -static packages :-)
> 
> So your patch essentially adds a call, that leaves a trace on the stack
> with our build-id. I guess that works as well.

Without that additional call this does not work and build_id selftest fails.

I.

Re: [PATCH net-next] selftests: bpf: modify urandom_read and link it non-statically

[Stanislav Fomichev]

On 03/15, Ivan Vecera wrote:
> 
> ----- Stanislav Fomichev <sdf@fomichev.me> wrote:
> > On 03/15, Ivan Vecera wrote:
> > > On 15. 03. 19 21:08, Stanislav Fomichev wrote:
> > > > On 03/15, Ivan Vecera wrote:
> > > > > After some experiences I found that urandom_read does not need to be
> > > > > linked statically. When the 'read' syscall call is moved to separate
> > > > > non-inlined function then bpf_get_stackid() is able to find
> > > > > the executable in stack trace and extract its build_id from it.
> > > > But why? Do you have some problems with it being linked statically?
> > > > 
> > > Dependency... you don't need to install static glibc to compile the bpf
> > > samples. Shared libc is available everytime.
> > Oh, the distros that do -devel _and_ -static packages :-)
> > 
> > So your patch essentially adds a call, that leaves a trace on the stack
> > with our build-id. I guess that works as well.
> 
> Without that additional call this does not work and build_id selftest fails.
Oh, yeah, I was just trying to clarify why it fails without -static and why
your patch makes it work for non-static :-)
You can put more details in the commit message; you'd have to resubmit
whenever net-next/bpf-next opens anyway.

> 
> I.

Re: [PATCH net-next] selftests: bpf: modify urandom_read and link it non-statically

[Alexei Starovoitov]

On Fri, Mar 15, 2019 at 1:04 PM Ivan Vecera <ivecera@redhat.com> wrote:
>
> After some experiences I found that urandom_read does not need to be
> linked statically. When the 'read' syscall call is moved to separate
> non-inlined function then bpf_get_stackid() is able to find
> the executable in stack trace and extract its build_id from it.
>
> Signed-off-by: Ivan Vecera <ivecera@redhat.com>

Applied. Thanks