[PATCH v3 bpf-next 0/2] bpf: add a ksym BPF iterator

[PATCH v3 bpf-next 0/2] bpf: add a ksym BPF iterator

[Alan Maguire]

a ksym BPF iterator would be useful as it would allow more flexible
interactions with kernel symbols than are currently supported; it could
for example create more efficient map representations for lookup,
speed up symbol resolution etc.

The idea was initially discussed here [1].

Changes since v2 [2]:

- set iter->show_value on initialization based on current creds
  and use it in selftest to determine if we show values
  (Yonghong, patches 1/2)
- inline iter registration into kallsyms_init (Yonghong, patch 1)

Changes since RFC [3]:

- change name of iterator (and associated structures/fields) to "ksym"
  (Andrii, patches 1, 2)
- remove dependency on CONFIG_PROC_FS; it was used for other BPF
  iterators, and I assumed it was needed because of seq ops but I
  don't think it is required on digging futher (Andrii, patch 1)

[1] https://lore.kernel.org/all/YjRPZj6Z8vuLeEZo@krava/
[2] https://lore.kernel.org/bpf/1656667620-18718-1-git-send-email-alan.maguire@oracle.com/
[3] https://lore.kernel.org/all/1656089118-577-1-git-send-email-alan.maguire@oracle.com/

Alan Maguire (2):
  bpf: add a ksym BPF iterator
  selftests/bpf: add a ksym iter subtest

 kernel/kallsyms.c                                 | 94 +++++++++++++++++++++++
 tools/testing/selftests/bpf/prog_tests/bpf_iter.c | 16 ++++
 tools/testing/selftests/bpf/progs/bpf_iter_ksym.c | 74 ++++++++++++++++++
 3 files changed, 184 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_ksym.c

-- 
1.8.3.1

[PATCH v3 bpf-next 1/2] bpf: add a ksym BPF iterator

[Alan Maguire]

add a "ksym" iterator which provides access to a "struct kallsym_iter"
for each symbol.  Intent is to support more flexible symbol parsing
as discussed in [1].

[1] https://lore.kernel.org/all/YjRPZj6Z8vuLeEZo@krava/

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 kernel/kallsyms.c | 94 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 94 insertions(+)

diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index fbdf8d3..9bd37be 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -30,6 +30,7 @@
 #include <linux/module.h>
 #include <linux/kernel.h>
 #include <linux/bsearch.h>
+#include <linux/btf_ids.h>
 
 /*
  * These will be re-linked against their real values
@@ -799,6 +800,87 @@ static int s_show(struct seq_file *m, void *p)
 	.show = s_show
 };
 
+#ifdef CONFIG_BPF_SYSCALL
+
+struct bpf_iter__ksym {
+	__bpf_md_ptr(struct bpf_iter_meta *, meta);
+	__bpf_md_ptr(struct kallsym_iter *, ksym);
+};
+
+static int ksym_prog_seq_show(struct seq_file *m, bool in_stop)
+{
+	struct bpf_iter__ksym ctx;
+	struct bpf_iter_meta meta;
+	struct bpf_prog *prog;
+
+	meta.seq = m;
+	prog = bpf_iter_get_info(&meta, in_stop);
+	if (!prog)
+		return 0;
+
+	ctx.meta = &meta;
+	ctx.ksym = m ? m->private : NULL;
+	return bpf_iter_run_prog(prog, &ctx);
+}
+
+static int bpf_iter_ksym_seq_show(struct seq_file *m, void *p)
+{
+	return ksym_prog_seq_show(m, false);
+}
+
+static void bpf_iter_ksym_seq_stop(struct seq_file *m, void *p)
+{
+	if (!p)
+		(void) ksym_prog_seq_show(m, true);
+	else
+		s_stop(m, p);
+}
+
+static const struct seq_operations bpf_iter_ksym_ops = {
+	.start = s_start,
+	.next = s_next,
+	.stop = bpf_iter_ksym_seq_stop,
+	.show = bpf_iter_ksym_seq_show,
+};
+
+static int bpf_iter_ksym_init(void *priv_data, struct bpf_iter_aux_info *aux)
+{
+	struct kallsym_iter *iter = priv_data;
+
+	reset_iter(iter, 0);
+
+	/* cache here as in kallsyms_open() case; use current process
+	 * credentials to tell BPF iterators if values should be shown.
+	 */
+	iter->show_value = kallsyms_show_value(current_cred());
+
+	return 0;
+}
+
+DEFINE_BPF_ITER_FUNC(ksym, struct bpf_iter_meta *meta, struct kallsym_iter *ksym)
+
+static const struct bpf_iter_seq_info ksym_iter_seq_info = {
+	.seq_ops		= &bpf_iter_ksym_ops,
+	.init_seq_private	= bpf_iter_ksym_init,
+	.fini_seq_private	= NULL,
+	.seq_priv_size		= sizeof(struct kallsym_iter),
+};
+
+static struct bpf_iter_reg ksym_iter_reg_info = {
+	.target                 = "ksym",
+	.ctx_arg_info_size	= 1,
+	.ctx_arg_info		= {
+		{ offsetof(struct bpf_iter__ksym, ksym),
+		  PTR_TO_BTF_ID_OR_NULL },
+	},
+	.seq_info		= &ksym_iter_seq_info,
+};
+
+BTF_ID_LIST(btf_ksym_iter_id)
+BTF_ID(struct, kallsym_iter)
+
+#endif /* CONFIG_BPF_SYSCALL */
+
 static inline int kallsyms_for_perf(void)
 {
 #ifdef CONFIG_PERF_EVENTS
@@ -885,6 +967,18 @@ const char *kdb_walk_kallsyms(loff_t *pos)
 static int __init kallsyms_init(void)
 {
 	proc_create("kallsyms", 0444, NULL, &kallsyms_proc_ops);
+#if defined(CONFIG_BPF_SYSCALL)
+	{
+		int ret;
+
+		ksym_iter_reg_info.ctx_arg_info[0].btf_id = *btf_ksym_iter_id;
+		ret = bpf_iter_reg_target(&ksym_iter_reg_info);
+		if (ret) {
+			pr_warn("Warning: could not register bpf ksym iterator: %d\n", ret);
+			return ret;
+		}
+	}
+#endif
 	return 0;
 }
 device_initcall(kallsyms_init);
-- 
1.8.3.1

[PATCH v3 bpf-next 2/2] selftests/bpf: add a ksym iter subtest

[Alan Maguire]

add subtest verifying BPF ksym iter behaviour.  The BPF ksym
iter program shows an example of dumping a format different to
/proc/kallsyms.  It adds KIND and MAX_SIZE fields which represent the
kind of symbol (core kernel, module, ftrace, bpf, or kprobe) and
the maximum size the symbol can be.  The latter is calculated from
the difference between current symbol value and the next symbol
value.

The key benefit for this iterator will likely be supporting in-kernel
data-gathering rather than dumping symbol details to userspace and
parsing the results.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Yonghong Song <yhs@fb.com>

---
 tools/testing/selftests/bpf/prog_tests/bpf_iter.c | 16 +++++
 tools/testing/selftests/bpf/progs/bpf_iter_ksym.c | 74 +++++++++++++++++++++++
 2 files changed, 90 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_ksym.c

diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
index 7ff5fa9..a33874b 100644
--- a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
@@ -27,6 +27,7 @@
 #include "bpf_iter_test_kern5.skel.h"
 #include "bpf_iter_test_kern6.skel.h"
 #include "bpf_iter_bpf_link.skel.h"
+#include "bpf_iter_ksym.skel.h"
 
 static int duration;
 
@@ -1120,6 +1121,19 @@ static void test_link_iter(void)
 	bpf_iter_bpf_link__destroy(skel);
 }
 
+static void test_ksym_iter(void)
+{
+	struct bpf_iter_ksym *skel;
+
+	skel = bpf_iter_ksym__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "bpf_iter_ksym__open_and_load"))
+		return;
+
+	do_dummy_read(skel->progs.dump_ksym);
+
+	bpf_iter_ksym__destroy(skel);
+}
+
 #define CMP_BUFFER_SIZE 1024
 static char task_vma_output[CMP_BUFFER_SIZE];
 static char proc_maps_output[CMP_BUFFER_SIZE];
@@ -1267,4 +1281,6 @@ void test_bpf_iter(void)
 		test_buf_neg_offset();
 	if (test__start_subtest("link-iter"))
 		test_link_iter();
+	if (test__start_subtest("ksym"))
+		test_ksym_iter();
 }
diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_ksym.c b/tools/testing/selftests/bpf/progs/bpf_iter_ksym.c
new file mode 100644
index 0000000..285c008
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/bpf_iter_ksym.c
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022, Oracle and/or its affiliates. */
+#include "bpf_iter.h"
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+unsigned long last_sym_value = 0;
+
+static inline char tolower(char c)
+{
+	if (c >= 'A' && c <= 'Z')
+		c += ('a' - 'A');
+	return c;
+}
+
+static inline char toupper(char c)
+{
+	if (c >= 'a' && c <= 'z')
+		c -= ('a' - 'A');
+	return c;
+}
+
+/* Dump symbols with max size; the latter is calculated by caching symbol N value
+ * and when iterating on symbol N+1, we can print max size of symbol N via
+ * address of N+1 - address of N.
+ */
+SEC("iter/ksym")
+int dump_ksym(struct bpf_iter__ksym *ctx)
+{
+	struct seq_file *seq = ctx->meta->seq;
+	struct kallsym_iter *iter = ctx->ksym;
+	__u32 seq_num = ctx->meta->seq_num;
+	unsigned long value;
+	char type;
+	int ret;
+
+	if (!iter)
+		return 0;
+
+	if (seq_num == 0) {
+		BPF_SEQ_PRINTF(seq, "ADDR TYPE NAME MODULE_NAME KIND MAX_SIZE\n");
+		return 0;
+	}
+	if (last_sym_value)
+		BPF_SEQ_PRINTF(seq, "0x%x\n", iter->value - last_sym_value);
+	else
+		BPF_SEQ_PRINTF(seq, "\n");
+
+	value = iter->show_value ? iter->value : 0;
+
+	last_sym_value = value;
+
+	type = iter->type;
+
+	if (iter->module_name[0]) {
+		type = iter->exported ? toupper(type) : tolower(type);
+		BPF_SEQ_PRINTF(seq, "0x%llx %c %s [ %s ] ",
+			       value, type, iter->name, iter->module_name);
+	} else {
+		BPF_SEQ_PRINTF(seq, "0x%llx %c %s ", value, type, iter->name);
+	}
+	if (!iter->pos_arch_end || iter->pos_arch_end > iter->pos)
+		BPF_SEQ_PRINTF(seq, "CORE ");
+	else if (!iter->pos_mod_end || iter->pos_mod_end > iter->pos)
+		BPF_SEQ_PRINTF(seq, "MOD ");
+	else if (!iter->pos_ftrace_mod_end || iter->pos_ftrace_mod_end > iter->pos)
+		BPF_SEQ_PRINTF(seq, "FTRACE_MOD ");
+	else if (!iter->pos_bpf_end || iter->pos_bpf_end > iter->pos)
+		BPF_SEQ_PRINTF(seq, "BPF ");
+	else
+		BPF_SEQ_PRINTF(seq, "KPROBE ");
+	return 0;
+}
-- 
1.8.3.1

Re: [PATCH v3 bpf-next 1/2] bpf: add a ksym BPF iterator

[Yonghong Song]

On 7/4/22 6:55 AM, Alan Maguire wrote:
> add a "ksym" iterator which provides access to a "struct kallsym_iter"
> for each symbol.  Intent is to support more flexible symbol parsing
> as discussed in [1].
> 
> [1] https://lore.kernel.org/all/YjRPZj6Z8vuLeEZo@krava/
> 
> Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>

Acked-by: Yonghong Song <yhs@fb.com>

Re: [PATCH v3 bpf-next 1/2] bpf: add a ksym BPF iterator

[Alexei Starovoitov]

On Mon, Jul 4, 2022 at 6:55 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>  static inline int kallsyms_for_perf(void)
>  {
>  #ifdef CONFIG_PERF_EVENTS
> @@ -885,6 +967,18 @@ const char *kdb_walk_kallsyms(loff_t *pos)
>  static int __init kallsyms_init(void)
>  {
>         proc_create("kallsyms", 0444, NULL, &kallsyms_proc_ops);
> +#if defined(CONFIG_BPF_SYSCALL)
> +       {
> +               int ret;
> +
> +               ksym_iter_reg_info.ctx_arg_info[0].btf_id = *btf_ksym_iter_id;
> +               ret = bpf_iter_reg_target(&ksym_iter_reg_info);
> +               if (ret) {
> +                       pr_warn("Warning: could not register bpf ksym iterator: %d\n", ret);
> +                       return ret;
> +               }
> +       }
> +#endif

The ifdef-s inside the function body are not pretty.
I feel the v2 version was cleaner.
static void __init bpf_ksym_iter_register()
were only missing late_initcall(bpf_ksym_iter_register);
to make it single #ifdef CONFIG_BPF_SYSCALL for everything.
wdyt?

Re: [PATCH v3 bpf-next 1/2] bpf: add a ksym BPF iterator

[Yonghong Song]

On 7/5/22 9:19 PM, Alexei Starovoitov wrote:
> On Mon, Jul 4, 2022 at 6:55 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>>   static inline int kallsyms_for_perf(void)
>>   {
>>   #ifdef CONFIG_PERF_EVENTS
>> @@ -885,6 +967,18 @@ const char *kdb_walk_kallsyms(loff_t *pos)
>>   static int __init kallsyms_init(void)
>>   {
>>          proc_create("kallsyms", 0444, NULL, &kallsyms_proc_ops);
>> +#if defined(CONFIG_BPF_SYSCALL)
>> +       {
>> +               int ret;
>> +
>> +               ksym_iter_reg_info.ctx_arg_info[0].btf_id = *btf_ksym_iter_id;
>> +               ret = bpf_iter_reg_target(&ksym_iter_reg_info);
>> +               if (ret) {
>> +                       pr_warn("Warning: could not register bpf ksym iterator: %d\n", ret);
>> +                       return ret;
>> +               }
>> +       }
>> +#endif
> 
> The ifdef-s inside the function body are not pretty.
> I feel the v2 version was cleaner.
> static void __init bpf_ksym_iter_register()
> were only missing late_initcall(bpf_ksym_iter_register);
> to make it single #ifdef CONFIG_BPF_SYSCALL for everything.
> wdyt?

Okay, a separate function is good too. Also agree with Alexei
that a separate init call bpf_ksym_iter_register() is needed
since we need late_initcall(...) instead of device_initcall(...)
to be consistent with other bpf_iter's.