[PATCHv2] btf_encoder: Match ftrace addresses within elf functions

[Jiri Olsa <jolsa@kernel.org>]

Currently when processing DWARF function, we check its entrypoint
against ftrace addresses, assuming that the ftrace address matches
with function's entrypoint.

This is not the case on some architectures as reported by Nathan
when building kernel on arm [1].

Fixing the check to take into account the whole function not
just the entrypoint.

Most of the is_ftrace_func code was contributed by Andrii.

[1] https://lore.kernel.org/bpf/20210209034416.GA1669105@ubuntu-m3-large-x86/
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
v2 changes:
  - update functions addr directly [Andrii]

 btf_encoder.c | 40 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index b124ec20a689..80e896961d4e 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -36,6 +36,7 @@ struct funcs_layout {
 struct elf_function {
 	const char	*name;
 	unsigned long	 addr;
+	unsigned long	 size;
 	unsigned long	 sh_addr;
 	bool		 generated;
 };
@@ -98,6 +99,7 @@ static int collect_function(struct btf_elf *btfe, GElf_Sym *sym,
 
 	functions[functions_cnt].name = name;
 	functions[functions_cnt].addr = elf_sym__value(sym);
+	functions[functions_cnt].size = elf_sym__size(sym);
 	functions[functions_cnt].sh_addr = sh.sh_addr;
 	functions[functions_cnt].generated = false;
 	functions_cnt++;
@@ -236,6 +238,39 @@ get_kmod_addrs(struct btf_elf *btfe, __u64 **paddrs, __u64 *pcount)
 	return 0;
 }
 
+static int is_ftrace_func(struct elf_function *func, __u64 *addrs, __u64 count)
+{
+	__u64 start = func->addr;
+	__u64 addr, end = func->addr + func->size;
+
+	/*
+	 * The invariant here is addr[r] that is the smallest address
+	 * that is >= than function start addr. Except the corner case
+	 * where there is no such r, but for that we have a final check
+	 * in the return.
+	 */
+	size_t l = 0, r = count - 1, m;
+
+	/* make sure we don't use invalid r */
+	if (count == 0)
+		return false;
+
+	while (l < r) {
+		m = l + (r - l) / 2;
+		addr = addrs[m];
+
+		if (addr >= start) {
+			/* we satisfy invariant, so tighten r */
+			r = m;
+		} else {
+			/* m is not good enough as l, maybe m + 1 will be */
+			l = m + 1;
+		}
+	}
+
+	return start <= addrs[r] && addrs[r] < end;
+}
+
 static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
 {
 	__u64 *addrs, count, i;
@@ -283,10 +318,11 @@ static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
 		 * functions[x]::addr is relative address within section
 		 * and needs to be relocated by adding sh_addr.
 		 */
-		__u64 addr = kmod ? func->addr + func->sh_addr : func->addr;
+		if (kmod)
+			func->addr += func->sh_addr;
 
 		/* Make sure function is within ftrace addresses. */
-		if (bsearch(&addr, addrs, count, sizeof(addrs[0]), addrs_cmp)) {
+		if (is_ftrace_func(func, addrs, count)) {
 			/*
 			 * We iterate over sorted array, so we can easily skip
 			 * not valid item and move following valid field into
-- 
2.29.2