Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

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

SNIP

quoted

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)