Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

From: Nathan Chancellor <nathan@kernel.org>
Date: 2021-02-11 17:26:49
Also in: bpf

On Thu, Feb 11, 2021 at 04:08:11PM +0100, Jiri Olsa wrote:

On Wed, Feb 10, 2021 at 09:13:47PM +0100, Jiri Olsa wrote:

quoted

On Wed, Feb 10, 2021 at 10:20:20AM -0800, Andrii Nakryiko wrote:

SNIP

quoted

but below is change for checking that ftrace addrs are within elf functions

seems to work in my tests, I'll run some more tests and send full patch

It seems unnecessarily convoluted. I was thinking about something like
this (the diff will totally be screwed up by gmail, and I haven't even
compiled it):

diff --git a/btf_encoder.c b/btf_encoder.c
index b124ec20a689..8162b238bd43 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c

@@ -236,6 +236,23 @@ get_kmod_addrs(struct btf_elf *btfe, __u64

**paddrs, __u64 *pcount)
        return 0;
 }

+struct func_seg { __u64 start; __u64 end; };
+
+static int func_exists(struct func_seg *segs, size_t len, __u64 addr)
+{
+       size_t l = 0, r = len - 1, m;
+
+       while (l < r) {
+               m = l + (r - l + 1) / 2;
+               if (segs[m].start <= addr)
+                       l = m;
+               else
+                       r = m - 1;
+       }
+
+       return segs[l].start <= addr && addr < segs[l].end;
+}
+
 static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
 {
        __u64 *addrs, count, i;

@@ -286,7 +303,7 @@ static int setup_functions(struct btf_elf *btfe,

struct funcs_layout *fl)
                __u64 addr = kmod ? func->addr + func->sh_addr : func->addr;

                /* Make sure function is within ftrace addresses. */
-               if (bsearch(&addr, addrs, count, sizeof(addrs[0]), addrs_cmp)) {
+               if (func_exists(addrs, count, addr))

you pass addrs in here, but you mean func_seg array
filled with elf functions start/end values, right?

quoted

                        /*
                         * We iterate over sorted array, so we can easily skip
                         * not valid item and move following valid field into


So the idea is to use address segments and check whether there is a
segment that overlaps with a given address by first binary searching
for a segment with the largest starting address that is <= addr. And
then just confirming that segment does overlap with the requested
address.

WDYT?

heya,
with your approach I ended up with change below, it gives me same
results as with the previous change

I think I'll separate the kmod bool address computation later on,
but I did not want to confuse this change for now

jirka

This still looks good against the original and reduced problematic
configurations for aarch64.

Tested-by: Nathan Chancellor <nathan@kernel.org>

quoted hunk ↗ jump to hunk

---

diff --git a/btf_encoder.c b/btf_encoder.c
index b124ec20a689..34df08f2fb4e 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	 end;
 	unsigned long	 sh_addr;
 	bool		 generated;
 };

@@ -44,7 +45,7 @@ static struct elf_function *functions;
 static int functions_alloc;
 static int functions_cnt;
 
-static int functions_cmp(const void *_a, const void *_b)
+static int functions_cmp_name(const void *_a, const void *_b)
 {
 	const struct elf_function *a = _a;
 	const struct elf_function *b = _b;

@@ -52,6 +53,16 @@ static int functions_cmp(const void *_a, const void *_b)
 	return strcmp(a->name, b->name);
 }
 
+static int functions_cmp_addr(const void *_a, const void *_b)
+{
+	const struct elf_function *a = _a;
+	const struct elf_function *b = _b;
+
+	if (a->addr == b->addr)
+		return 0;
+	return a->addr < b->addr ? -1 : 1;
+}
+
 static void delete_functions(void)
 {
 	free(functions);

@@ -98,6 +109,7 @@ 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].end = (__u64) -1;
 	functions[functions_cnt].sh_addr = sh.sh_addr;
 	functions[functions_cnt].generated = false;
 	functions_cnt++;

@@ -236,6 +248,40 @@ 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, bool kmod)
+{
+	/*
+	 * For vmlinux image both addrs[x] and functions[x]::addr
+	 * values are final address and are comparable.
+	 *
+	 * For kernel module addrs[x] is final address, but
+	 * functions[x]::addr is relative address within section
+	 * and needs to be relocated by adding sh_addr.
+	 */
+	__u64 start = kmod ? func->addr + func->sh_addr : func->addr;
+	__u64 end   = kmod ? func->end + func->sh_addr : func->end;
+
+	size_t l = 0, r = count - 1, m;
+	__u64 addr = 0;
+
+	while (l < r) {
+		m = l + (r - l + 1) / 2;
+		addr = addrs[m];
+
+		if (start <= addr && addr < end)
+			return true;
+
+		if (start <= addr)
+			r = m - 1;
+		else
+			l = m;
+	}
+
+	addr = addrs[l];
+	return start <= addr && addr < end;
+}
+
 static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
 {
 	__u64 *addrs, count, i;

@@ -267,7 +313,7 @@ static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
 	}
 
 	qsort(addrs, count, sizeof(addrs[0]), addrs_cmp);
-	qsort(functions, functions_cnt, sizeof(functions[0]), functions_cmp);
+	qsort(functions, functions_cnt, sizeof(functions[0]), functions_cmp_addr);
 
 	/*
 	 * Let's got through all collected functions and filter

@@ -275,18 +321,12 @@ static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
 	 */
 	for (i = 0; i < functions_cnt; i++) {
 		struct elf_function *func = &functions[i];
-		/*
-		 * For vmlinux image both addrs[x] and functions[x]::addr
-		 * values are final address and are comparable.
-		 *
-		 * For kernel module addrs[x] is final address, but
-		 * 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 (i + 1 < functions_cnt)
+			func->end = functions[i + 1].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, kmod)) {
 			/*
 			 * We iterate over sorted array, so we can easily skip
 			 * not valid item and move following valid field into

@@ -303,6 +343,8 @@ static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
 
 	if (btf_elf__verbose)
 		printf("Found %d functions!\n", functions_cnt);
+
+	qsort(functions, functions_cnt, sizeof(functions[0]), functions_cmp_name);
 	return 0;
 }

@@ -312,7 +354,7 @@ static struct elf_function *find_function(const struct btf_elf *btfe,
 	struct elf_function key = { .name = name };
 
 	return bsearch(&key, functions, functions_cnt, sizeof(functions[0]),
-		       functions_cmp);
+		       functions_cmp_name);
 }
 
 static bool btf_name_char_ok(char c, bool first)

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