Re: [PATCH v2 bpf-next 15/17] selftests/bpf: add function linking selftest

On Fri, Apr 23, 2021 at 10:35 AM Yonghong Song [off-list ref] wrote:

On 4/23/21 10:18 AM, Andrii Nakryiko wrote:

quoted

On Thu, Apr 22, 2021 at 5:50 PM Yonghong Song [off-list ref] wrote:

quoted

On 4/16/21 1:24 PM, Andrii Nakryiko wrote:

quoted

Add selftest validating various aspects of statically linking functions:
    - no conflicts and correct resolution for name-conflicting static funcs;
    - correct resolution of extern functions;
    - correct handling of weak functions, both resolution itself and libbpf's
      handling of unused weak function that "lost" (it leaves gaps in code with
      no ELF symbols);
    - correct handling of hidden visibility to turn global function into
      "static" for the purpose of BPF verification.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

Ack with a small nit below.

Acked-by: Yonghong Song <redacted>

quoted

---
   tools/testing/selftests/bpf/Makefile          |  3 +-
   .../selftests/bpf/prog_tests/linked_funcs.c   | 42 +++++++++++
   .../selftests/bpf/progs/linked_funcs1.c       | 73 +++++++++++++++++++
   .../selftests/bpf/progs/linked_funcs2.c       | 73 +++++++++++++++++++
   4 files changed, 190 insertions(+), 1 deletion(-)
   create mode 100644 tools/testing/selftests/bpf/prog_tests/linked_funcs.c
   create mode 100644 tools/testing/selftests/bpf/progs/linked_funcs1.c
   create mode 100644 tools/testing/selftests/bpf/progs/linked_funcs2.c

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 666b462c1218..427ccfec1a6a 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile

@@ -308,9 +308,10 @@ endef

   SKEL_BLACKLIST := btf__% test_pinning_invalid.c test_sk_assign.c

-LINKED_SKELS := test_static_linked.skel.h
+LINKED_SKELS := test_static_linked.skel.h linked_funcs.skel.h

   test_static_linked.skel.h-deps := test_static_linked1.o test_static_linked2.o
+linked_funcs.skel.h-deps := linked_funcs1.o linked_funcs2.o

   LINKED_BPF_SRCS := $(patsubst %.o,%.c,$(foreach skel,$(LINKED_SKELS),$($(skel)-deps)))

diff --git a/tools/testing/selftests/bpf/prog_tests/linked_funcs.c b/tools/testing/selftests/bpf/prog_tests/linked_funcs.c
new file mode 100644
index 000000000000..03bf8ef131ce
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/linked_funcs.c

@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+
+#include <test_progs.h>
+#include <sys/syscall.h>
+#include "linked_funcs.skel.h"
+
+void test_linked_funcs(void)
+{
+     int err;
+     struct linked_funcs *skel;
+
+     skel = linked_funcs__open();
+     if (!ASSERT_OK_PTR(skel, "skel_open"))
+             return;
+
+     skel->rodata->my_tid = syscall(SYS_gettid);
+     skel->rodata->syscall_id = SYS_getpgid;
+
+     err = linked_funcs__load(skel);
+     if (!ASSERT_OK(err, "skel_load"))
+             goto cleanup;
+
+     err = linked_funcs__attach(skel);
+     if (!ASSERT_OK(err, "skel_attach"))
+             goto cleanup;
+
+     /* trigger */
+     syscall(SYS_getpgid);
+
+     ASSERT_EQ(skel->bss->output_val1, 2000 + 2000, "output_val1");
+     ASSERT_EQ(skel->bss->output_ctx1, SYS_getpgid, "output_ctx1");
+     ASSERT_EQ(skel->bss->output_weak1, 42, "output_weak1");
+
+     ASSERT_EQ(skel->bss->output_val2, 2 * 1000 + 2 * (2 * 1000), "output_val2");
+     ASSERT_EQ(skel->bss->output_ctx2, SYS_getpgid, "output_ctx2");
+     /* output_weak2 should never be updated */
+     ASSERT_EQ(skel->bss->output_weak2, 0, "output_weak2");
+
+cleanup:
+     linked_funcs__destroy(skel);
+}

diff --git a/tools/testing/selftests/bpf/progs/linked_funcs1.c b/tools/testing/selftests/bpf/progs/linked_funcs1.c
new file mode 100644
index 000000000000..cc621d4e4d82
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/linked_funcs1.c

@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+/* weak and shared between two files */
+const volatile int my_tid __weak = 0;
+const volatile long syscall_id __weak = 0;

Since the new compiler (llvm13) is recommended for this patch set.
We can simplify the above two definition with
    int my_tid __weak;
    long syscall_id __weak;
The same for the other file.

This is not about old vs new compilers. I wanted to use .rodata
variables, but I'll switch to .bss, no problem.

I see. You can actually hone one "const volatile ing my_tid __weak = 0"
and another "long syscall_id __weak". This way, you will be able to
test both .rodata and .bss section.

I wonder if you meant to have one my_tid __weak in .bss and another
my_tid __weak in .rodata. Or just my_tid in .bss and syscall_id in
.rodata?

If the former (mixing ELF sections across definitions of the same
symbol), then it's disallowed right now. libbpf will error out on
mismatched sections. I tested this with normal compilation, it does
work and the final section is the section of the winner.

But I think that's quite confusing, actually, so I'm going to leave it
disallowed for now. E.g., if one file expects a read-write variable
and another expects that same variable to be read-only, and the winner
ends up being read-only one, then the file expecting read-write will
essentially have incorrect code (and will be rejected by BPF verifier,
if anything attempts to write). So I think it's better to reject it at
the linking time.

But I'll do one (my_tid) as .bss, and another (syscall_id) as .rodata.

quoted

quoted

But I am also okay with the current form
to *satisfy* llvm10 some people may still use.

quoted

+
+int output_val1 = 0;
+int output_ctx1 = 0;
+int output_weak1 = 0;
+
+/* same "subprog" name in all files, but it's ok because they all are static */
+static __noinline int subprog(int x)
+{
+     /* but different formula */
+     return x * 1;
+}
+
+/* Global functions can't be void */
+int set_output_val1(int x)
+{
+     output_val1 = x + subprog(x);
+     return x;
+}
+
+/* This function can't be verified as global, as it assumes raw_tp/sys_enter
+ * context and accesses syscall id (second argument). So we mark it as
+ * __hidden, so that libbpf will mark it as static in the final object file,
+ * right before verifying it in the kernel.
+ *
+ * But we don't mark it as __hidden here, rather at extern site. __hidden is
+ * "contaminating" visibility, so it will get propagated from either extern or
+ * actual definition (including from the losing __weak definition).
+ */
+void set_output_ctx1(__u64 *ctx)
+{
+     output_ctx1 = ctx[1]; /* long id, same as in BPF_PROG below */
+}
+
+/* this weak instance should win because it's the first one */
+__weak int set_output_weak(int x)
+{
+     output_weak1 = x;
+     return x;
+}
+
+extern int set_output_val2(int x);
+
+/* here we'll force set_output_ctx2() to be __hidden in the final obj file */
+__hidden extern void set_output_ctx2(__u64 *ctx);
+

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