[PATCH bpf-next v5] selftests/bpf: trace_helpers.c: optimize kallsyms cache

[Rong Tao]

From: Rong Tao <rongtao@xxxxxxxx>

Static ksyms often have problems because the number of symbols exceeds the
MAX_SYMS limit. Like changing the MAX_SYMS from 300000 to 400000 in
commit e76a014334a6("selftests/bpf: Bump and validate MAX_SYMS") solves
the problem somewhat, but it's not the perfect way.

This commit uses dynamic memory allocation, which completely solves the
problem caused by the limitation of the number of kallsyms.

Acked-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
Signed-off-by: Rong Tao <rongtao@xxxxxxxx>
---
v5: Release the allocated memory once the load_kallsyms_refresh() upon error
    given it's dynamically allocated.
v4: https://lore.kernel.org/lkml/tencent_59C74613113F0C728524B2A82FE5540A5E09@xxxxxx/
    Make sure most cases we don't need the realloc() path to begin with,
    and check strdup() return value.
v3: https://lore.kernel.org/lkml/tencent_50B4B2622FE7546A5FF9464310650C008509@xxxxxx/
    Do not use structs and judge ksyms__add_symbol function return value.
v2: https://lore.kernel.org/lkml/tencent_B655EE5E5D463110D70CD2846AB3262EED09@xxxxxx/
    Do the usual len/capacity scheme here to amortize the cost of realloc, and
    don't free symbols.
v1: https://lore.kernel.org/lkml/tencent_AB461510B10CD484E0B2F62E3754165F2909@xxxxxx/
---
 tools/testing/selftests/bpf/trace_helpers.c | 62 +++++++++++++++++----
 1 file changed, 52 insertions(+), 10 deletions(-)

diff --git a/tools/testing/selftests/bpf/trace_helpers.c b/tools/testing/selftests/bpf/trace_helpers.c
index f83d9f65c65b..0053ba22f0cb 100644
--- a/tools/testing/selftests/bpf/trace_helpers.c
+++ b/tools/testing/selftests/bpf/trace_helpers.c
@@ -18,10 +18,47 @@
 #define TRACEFS_PIPE	"/sys/kernel/tracing/trace_pipe"
 #define DEBUGFS_PIPE	"/sys/kernel/debug/tracing/trace_pipe"
 
-#define MAX_SYMS 400000
-static struct ksym syms[MAX_SYMS];
+static struct ksym *syms;
+static int sym_cap;
 static int sym_cnt;
 
+static int ksyms__add_symbol(const char *name, unsigned long addr)
+{
+	void *tmp;
+	unsigned int new_cap;
+
+	if (sym_cnt + 1 > sym_cap) {
+		new_cap = sym_cap * 4 / 3;
+		tmp = realloc(syms, sizeof(struct ksym) * new_cap);
+		if (!tmp)
+			return -ENOMEM;
+		syms = tmp;
+		sym_cap = new_cap;
+	}
+
+	tmp = strdup(name);
+	if (!tmp)
+		return -ENOMEM;
+	syms[sym_cnt].addr = addr;
+	syms[sym_cnt].name = tmp;
+
+	sym_cnt++;
+
+	return 0;
+}
+
+static void ksyms__free(void)
+{
+	unsigned int i;
+
+	if (!syms)
+		return;
+
+	for (i = 0; i < sym_cnt; i++)
+		free(syms[i].name);
+	free(syms);
+}
+
 static int ksym_cmp(const void *p1, const void *p2)
 {
 	return ((struct ksym *)p1)->addr - ((struct ksym *)p2)->addr;
@@ -33,9 +70,14 @@ int load_kallsyms_refresh(void)
 	char func[256], buf[256];
 	char symbol;
 	void *addr;
-	int i = 0;
+	int ret;
 
+	/* Make sure most cases we don't need the realloc() path to begin with */
+	sym_cap = 400000;
 	sym_cnt = 0;
+	syms = malloc(sizeof(struct ksym) * sym_cap);
+	if (!syms)
+		return -ENOMEM;
 
 	f = fopen("/proc/kallsyms", "r");
 	if (!f)
@@ -46,17 +88,17 @@ int load_kallsyms_refresh(void)
 			break;
 		if (!addr)
 			continue;
-		if (i >= MAX_SYMS)
-			return -EFBIG;
-
-		syms[i].addr = (long) addr;
-		syms[i].name = strdup(func);
-		i++;
+		ret = ksyms__add_symbol(func, (unsigned long)addr);
+		if (ret)
+			goto error;
 	}
 	fclose(f);
-	sym_cnt = i;
 	qsort(syms, sym_cnt, sizeof(struct ksym), ksym_cmp);
 	return 0;
+
+error:
+	ksyms__free();
+	return ret;
 }
 
 int load_kallsyms(void)
-- 
2.39.3