Subject: Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

From: Jiri Olsa <[email-masked]>
Date: Fri, 12 Feb 2021 22:29:18 +0100

On Fri, Feb 12, 2021 at 11:22:41AM -0800, Andrii Nakryiko wrote:

SNIP

> > +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);
> 
> All looks good except this. We don't rely on functions being sorted in
> ascending start addr order, do we? If not, just drop this, no need to
> slow down the process.

right, it's not needed when we use st_size for function size

thanks,
jirka