Merge branch 'libbpf: support custom .rodata.*/.data.* sections'

Andrii Nakryiko says:

====================

This patch set refactors internals of libbpf to enable support for multiple
custom .rodata.* and .data.* sections. Each such section is backed by its own
BPF_MAP_TYPE_ARRAY, memory-mappable just like .rodata/.data. This is not
extended to .bss because .bss is not a great name, it is generated by compiler
with name that reflects completely irrelevant historical implementation
details. Given that users have to annotate their variables with
SEC(".data.my_sec") explicitly, standardizing on .rodata. and .data. prefixes
makes more sense and keeps things simpler.

Additionally, this patch set makes it simpler to work with those special
internal maps by allowing to look them up by their full ELF section name.

Patch #1 is a preparatory patch that deprecates one libbpf API and moves
custom logic into libbpf.c, where it's used. This code is later refactored
with the rest of libbpf.c logic to support multiple data section maps.

See individual patches for all the details.

For new custom "dot maps", their full ELF section names are used as the names
that are sent into the kernel. Object name isn't prepended like for
.data/.rodata/.bss. The reason is that with longer custom names, there isn't
much space left for object name anyways. Also, if BTF is supported,
btf_value_type_id points to DATASEC BTF type, which contains full original ELF
name of the section, so tools like bpftool could use that to recover full
name. This patch set doesn't add this logic yet, this is left for follow up
patches.

One interesting possibility that is now open by these changes is that it's
possible to do:

    bpf_trace_printk("My fmt %s", sizeof("My fmt %s"), "blah");

and it will work as expected. I haven't updated libbpf-provided helpers in
bpf_helpers.h for snprintf, seq_printf, and printk, because using
`static const char ___fmt[] = fmt;` trick is still efficient and doesn't fill
out the buffer at runtime (no copying). But we might consider updating them in
the future, especially with the array check that Kumar proposed (see [0]).

  [0] https://lore.kernel.org/bpf/20211012041524.udytbr2xs5wid6x2@apollo.localdomain/

v1->v2:
  - don't prepend object name for new dot maps;
  - add __read_mostly example in selftests (Daniel).
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

7 files changed, 43 insertions, 7 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/core_autosize.c b/tools/testing/selftests/bpf/prog_tests/core_autosize.c
index 3d4b2a358d47..2a0dac6394ef 100644
--- a/tools/testing/selftests/bpf/prog_tests/core_autosize.c
+++ b/tools/testing/selftests/bpf/prog_tests/core_autosize.c
@@ -163,7 +163,7 @@ void test_core_autosize(void)
 
 	usleep(1);
 
-	bss_map = bpf_object__find_map_by_name(skel->obj, "test_cor.bss");
+	bss_map = bpf_object__find_map_by_name(skel->obj, ".bss");
 	if (!ASSERT_OK_PTR(bss_map, "bss_map_find"))
 		goto cleanup;
 
diff --git a/tools/testing/selftests/bpf/prog_tests/core_reloc.c b/tools/testing/selftests/bpf/prog_tests/core_reloc.c
index 763302e63a29..cc50f8feeca3 100644
--- a/tools/testing/selftests/bpf/prog_tests/core_reloc.c
+++ b/tools/testing/selftests/bpf/prog_tests/core_reloc.c
@@ -867,7 +867,7 @@ void test_core_reloc(void)
 			goto cleanup;
 		}
 
-		data_map = bpf_object__find_map_by_name(obj, "test_cor.bss");
+		data_map = bpf_object__find_map_by_name(obj, ".bss");
 		if (CHECK(!data_map, "find_data_map", "data map not found\n"))
 			goto cleanup;
 
diff --git a/tools/testing/selftests/bpf/prog_tests/global_data.c b/tools/testing/selftests/bpf/prog_tests/global_data.c
index 9efa7e50eab2..afd8639f9a94 100644
--- a/tools/testing/selftests/bpf/prog_tests/global_data.c
+++ b/tools/testing/selftests/bpf/prog_tests/global_data.c
@@ -103,11 +103,18 @@ static void test_global_data_struct(struct bpf_object *obj, __u32 duration)
 static void test_global_data_rdonly(struct bpf_object *obj, __u32 duration)
 {
 	int err = -ENOMEM, map_fd, zero = 0;
-	struct bpf_map *map;
+	struct bpf_map *map, *map2;
 	__u8 *buff;
 
 	map = bpf_object__find_map_by_name(obj, "test_glo.rodata");
-	if (CHECK_FAIL(!map || !bpf_map__is_internal(map)))
+	if (!ASSERT_OK_PTR(map, "map"))
+		return;
+	if (!ASSERT_TRUE(bpf_map__is_internal(map), "is_internal"))
+		return;
+
+	/* ensure we can lookup internal maps by their ELF names */
+	map2 = bpf_object__find_map_by_name(obj, ".rodata");
+	if (!ASSERT_EQ(map, map2, "same_maps"))
 		return;
 
 	map_fd = bpf_map__fd(map);
diff --git a/tools/testing/selftests/bpf/prog_tests/global_data_init.c b/tools/testing/selftests/bpf/prog_tests/global_data_init.c
index ee46b11f1f9a..1db86eab101b 100644
--- a/tools/testing/selftests/bpf/prog_tests/global_data_init.c
+++ b/tools/testing/selftests/bpf/prog_tests/global_data_init.c
@@ -16,7 +16,7 @@ void test_global_data_init(void)
 	if (CHECK_FAIL(err))
 		return;
 
-	map = bpf_object__find_map_by_name(obj, "test_glo.rodata");
+	map = bpf_object__find_map_by_name(obj, ".rodata");
 	if (CHECK_FAIL(!map || !bpf_map__is_internal(map)))
 		goto out;
 
diff --git a/tools/testing/selftests/bpf/prog_tests/kfree_skb.c b/tools/testing/selftests/bpf/prog_tests/kfree_skb.c
index 032a322d51f2..01e51d16c8b8 100644
--- a/tools/testing/selftests/bpf/prog_tests/kfree_skb.c
+++ b/tools/testing/selftests/bpf/prog_tests/kfree_skb.c
@@ -93,7 +93,7 @@ void serial_test_kfree_skb(void)
 	if (CHECK(!fexit, "find_prog", "prog eth_type_trans not found\n"))
 		goto close_prog;
 
-	global_data = bpf_object__find_map_by_name(obj2, "kfree_sk.bss");
+	global_data = bpf_object__find_map_by_name(obj2, ".bss");
 	if (CHECK(!global_data, "find global data", "not found\n"))
 		goto close_prog;
 
diff --git a/tools/testing/selftests/bpf/prog_tests/rdonly_maps.c b/tools/testing/selftests/bpf/prog_tests/rdonly_maps.c
index 5f9eaa3ab584..fd5d2ddfb062 100644
--- a/tools/testing/selftests/bpf/prog_tests/rdonly_maps.c
+++ b/tools/testing/selftests/bpf/prog_tests/rdonly_maps.c
@@ -37,7 +37,7 @@ void test_rdonly_maps(void)
 	if (CHECK(err, "obj_load", "err %d errno %d\n", err, errno))
 		goto cleanup;
 
-	bss_map = bpf_object__find_map_by_name(obj, "test_rdo.bss");
+	bss_map = bpf_object__find_map_by_name(obj, ".bss");
 	if (CHECK(!bss_map, "find_bss_map", "failed\n"))
 		goto cleanup;
 
diff --git a/tools/testing/selftests/bpf/prog_tests/skeleton.c b/tools/testing/selftests/bpf/prog_tests/skeleton.c
index fe1e204a65c6..180afd632f4c 100644
--- a/tools/testing/selftests/bpf/prog_tests/skeleton.c
+++ b/tools/testing/selftests/bpf/prog_tests/skeleton.c
@@ -16,10 +16,13 @@ void test_skeleton(void)
 	struct test_skeleton* skel;
 	struct test_skeleton__bss *bss;
 	struct test_skeleton__data *data;
+	struct test_skeleton__data_dyn *data_dyn;
 	struct test_skeleton__rodata *rodata;
+	struct test_skeleton__rodata_dyn *rodata_dyn;
 	struct test_skeleton__kconfig *kcfg;
 	const void *elf_bytes;
 	size_t elf_bytes_sz = 0;
+	int i;
 
 	skel = test_skeleton__open();
 	if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
@@ -30,7 +33,12 @@ void test_skeleton(void)
 
 	bss = skel->bss;
 	data = skel->data;
+	data_dyn = skel->data_dyn;
 	rodata = skel->rodata;
+	rodata_dyn = skel->rodata_dyn;
+
+	ASSERT_STREQ(bpf_map__name(skel->maps.rodata_dyn), ".rodata.dyn", "rodata_dyn_name");
+	ASSERT_STREQ(bpf_map__name(skel->maps.data_dyn), ".data.dyn", "data_dyn_name");
 
 	/* validate values are pre-initialized correctly */
 	CHECK(data->in1 != -1, "in1", "got %d != exp %d\n", data->in1, -1);
@@ -46,6 +54,12 @@ void test_skeleton(void)
 	CHECK(rodata->in.in6 != 0, "in6", "got %d != exp %d\n", rodata->in.in6, 0);
 	CHECK(bss->out6 != 0, "out6", "got %d != exp %d\n", bss->out6, 0);
 
+	ASSERT_EQ(rodata_dyn->in_dynarr_sz, 0, "in_dynarr_sz");
+	for (i = 0; i < 4; i++)
+		ASSERT_EQ(rodata_dyn->in_dynarr[i], -(i + 1), "in_dynarr");
+	for (i = 0; i < 4; i++)
+		ASSERT_EQ(data_dyn->out_dynarr[i], i + 1, "out_dynarr");
+
 	/* validate we can pre-setup global variables, even in .bss */
 	data->in1 = 10;
 	data->in2 = 11;
@@ -53,6 +67,10 @@ void test_skeleton(void)
 	bss->in4 = 13;
 	rodata->in.in6 = 14;
 
+	rodata_dyn->in_dynarr_sz = 4;
+	for (i = 0; i < 4; i++)
+		rodata_dyn->in_dynarr[i] = i + 10;
+
 	err = test_skeleton__load(skel);
 	if (CHECK(err, "skel_load", "failed to load skeleton: %d\n", err))
 		goto cleanup;
@@ -64,6 +82,10 @@ void test_skeleton(void)
 	CHECK(bss->in4 != 13, "in4", "got %lld != exp %lld\n", bss->in4, 13LL);
 	CHECK(rodata->in.in6 != 14, "in6", "got %d != exp %d\n", rodata->in.in6, 14);
 
+	ASSERT_EQ(rodata_dyn->in_dynarr_sz, 4, "in_dynarr_sz");
+	for (i = 0; i < 4; i++)
+		ASSERT_EQ(rodata_dyn->in_dynarr[i], i + 10, "in_dynarr");
+
 	/* now set new values and attach to get them into outX variables */
 	data->in1 = 1;
 	data->in2 = 2;
@@ -73,6 +95,8 @@ void test_skeleton(void)
 	bss->in5.b = 6;
 	kcfg = skel->kconfig;
 
+	skel->data_read_mostly->read_mostly_var = 123;
+
 	err = test_skeleton__attach(skel);
 	if (CHECK(err, "skel_attach", "skeleton attach failed: %d\n", err))
 		goto cleanup;
@@ -93,6 +117,11 @@ void test_skeleton(void)
 	CHECK(bss->kern_ver != kcfg->LINUX_KERNEL_VERSION, "ext2",
 	      "got %d != exp %d\n", bss->kern_ver, kcfg->LINUX_KERNEL_VERSION);
 
+	for (i = 0; i < 4; i++)
+		ASSERT_EQ(data_dyn->out_dynarr[i], i + 10, "out_dynarr");
+
+	ASSERT_EQ(skel->bss->out_mostly_var, 123, "out_mostly_var");
+
 	elf_bytes = test_skeleton__elf_bytes(&elf_bytes_sz);
 	ASSERT_OK_PTR(elf_bytes, "elf_bytes");
 	ASSERT_GE(elf_bytes_sz, 0, "elf_bytes_sz");