Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

Jiri Olsa
2021-02-11T15:18:38Z

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)