Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Feb 10, 2021 at 09:13:47PM +0100, Jiri Olsa wrote:
> On Wed, Feb 10, 2021 at 10:20:20AM -0800, Andrii Nakryiko wrote:
> 
> SNIP
> 
> > > 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?
> 
> >                         /*
> >                          * 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


---
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)