On Fri, 25 Mar 2022, Andrii Nakryiko wrote:
> Extend urandom_read helper binary to include USDTs of 4 combinations:
> semaphore/semaphoreless (refcounted and non-refcounted) and based in
> executable or shared library. We also extend urandom_read with ability
> to report it's own PID to parent process and wait for parent process to
> ready itself up for tracing urandom_read. We utilize popen() and
> underlying pipe properties for proper signaling.
>
> Once urandom_read is ready, we add few tests to validate that libbpf's
> USDT attachment handles all the above combinations of semaphore (or lack
> of it) and static or shared library USDTs. Also, we validate that libbpf
> handles shared libraries both with PID filter and without one (i.e., -1
> for PID argument).
>
> Having the shared library case tested with and without PID is important
> because internal logic differs on kernels that don't support BPF
> cookies. On such older kernels, attaching to USDTs in shared libraries
> without specifying concrete PID doesn't work in principle, because it's
> impossible to determine shared library's load address to derive absolute
> IPs for uprobe attachments. Without absolute IPs, it's impossible to
> perform correct look up of USDT spec based on uprobe's absolute IP (the
> only kind available from BPF at runtime). This is not the problem on
> newer kernels with BPF cookie as we don't need IP-to-ID lookup because
> BPF cookie value *is* spec ID.
>
> So having those two situations as separate subtests is good because
> libbpf CI is able to test latest selftests against old kernels (e.g.,
> 4.9 and 5.5), so we'll be able to disable PID-less shared lib attachment
> for old kernels, but will still leave PID-specific one enabled to validate
> this legacy logic is working correctly.
>
> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
haven't looked at this in depth yet, but hit a compilation error on
aarch64:
LIB liburandom_read.so
/usr/bin/ld: /tmp/ccNy8cuv.o: relocation R_AARCH64_ADR_PREL_PG_HI21
against symbol `urandlib_read_with_sema_semaphore' which may bind
externally can not be used when making a shared object; recompile with
-fPIC
/tmp/ccNy8cuv.o: In function `urandlib_read_with_sema':
/home/opc/src/bpf-next/tools/testing/selftests/bpf/urandom_read_lib1.c:12:(.text+0x10):
dangerous relocation: unsupported relocation
collect2: error: ld returned 1 exit status
make: *** [Makefile:173:
/home/opc/src/bpf-next/tools/testing/selftests/bpf/liburandom_read.so]
Error 1
following did fix it:
diff --git a/tools/testing/selftests/bpf/Makefile
b/tools/testing/selftests/bpf/Makefile
index 58da22c019a8..c89e2948276b 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -170,7 +170,7 @@ $(OUTPUT)/%:%.c
$(OUTPUT)/liburandom_read.so: urandom_read_lib1.c urandom_read_lib2.c
$(call msg,LIB,,$@)
- $(Q)$(CC) $(CFLAGS) $(LDFLAGS) $^ $(LDLIBS) --shared -o $@
+ $(Q)$(CC) $(CFLAGS) -fPIC $(LDFLAGS) $^ $(LDLIBS) --shared -o $@
$(OUTPUT)/urandom_read: urandom_read.c urandom_read_aux.c
$(OUTPUT)/liburandom_read.so
$(call msg,BINARY,,$@)
