Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

Jiri Olsa
2021-02-12T16:42:21Z

On Thu, Feb 11, 2021 at 11:59:02AM -0800, Andrii Nakryiko wrote:

SNIP

>
> So in my previous example I assumed we have address ranges for ftrace
> section, which is exactly the opposite from what we have. So this
> binary search should be a bit different. start <= addr seems wrong
> here as well.
>
> The invariant here should be that addr[r] is the smallest address that
> is >= than function start addr, right? Except the corner case where
> there is no such r, but for that we have a final check in the return
> below. If you wanted to use index l, you'd need to change the
> invariant to find the largest addr, such that it is < end, but that
> seems a bit convoluted.
>
> So, with that, I think it should be like this:
>
> size_t l = 0, r = count - 1, m;
>
> /* make sure we don't use invalid r */
> if (count == 0) return false;
>
> while (l < r) {
> /* note no +1 in this case, it's so that at the end, when you
> * have, say, l = 0, and r = 1, you try l first, not r.
> * Otherwise you might end in in the infinite loop when r never == l.
> */
> 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;
>
>
> So, basically, r is maintained as a valid index always, while we
> constantly try to tighten the l.
>
> Does this make sense?

another take ;-)

jirka

---
diff --git a/btf_encoder.c b/btf_encoder.c
index b124ec20a689..20a93ed60e52 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;
};
@@ -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].size = elf_sym__size(sym);
functions[functions_cnt].sh_addr = sh.sh_addr;
functions[functions_cnt].generated = false;
functions_cnt++;
@@ -236,6 +248,48 @@ 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 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;
@@ -267,7 +321,7 @@ 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 +329,9 @@ 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;

/* 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 +348,8 @@ 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 +359,7 @@ 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)