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

From: Jiri Olsa <[email-masked]>
Date: Wed, 10 Feb 2021 21:13:43 +0100

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?

I liked the approach below because it got rid of that bsearch ;-)

but yea, yours seems to be less code chage and staighforward

jirka

> 
> >
> > jirka
> >
> >
> > ---
> > diff --git a/btf_encoder.c b/btf_encoder.c
> > index b124ec20a689..548a12847f99 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++;
> > @@ -236,9 +248,25 @@ get_kmod_addrs(struct btf_elf *btfe, __u64 **paddrs, __u64 *pcount)
> >         return 0;
> >  }
> >
> > +static bool is_addr_in_func(__u64 addr, struct elf_function *func, 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;
> > +
> > +       return start <= addr && addr < end;
> > +}
> > +
> >  static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
> >  {
> > -       __u64 *addrs, count, i;
> > +       __u64 *addrs, count, i_func, i_addr;
> >         int functions_valid = 0;
> >         bool kmod = false;
> >
> > @@ -266,43 +294,62 @@ static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
> >                 return 0;
> >         }
> >
> > -       qsort(addrs, count, sizeof(addrs[0]), addrs_cmp);
> > -       qsort(functions, functions_cnt, sizeof(functions[0]), functions_cmp);
> > -
> >         /*
> > -        * Let's got through all collected functions and filter
> > -        * out those that are not in ftrace.
> > +        * Sort both functions and addrs so we can iterate
> > +        * both of them simultaneously and found matching
> > +        * func/addr pairs.
> >          */
> > -       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;
> > +       qsort(addrs, count, sizeof(addrs[0]), addrs_cmp);
> > +       qsort(functions, functions_cnt, sizeof(functions[0]), functions_cmp_addr);
> > +
> > +       for (i_func = 0, i_addr = 0; i_func < functions_cnt; i_func++) {
> > +               struct elf_function *func = &functions[i_func];
> > +
> > +               if (i_func + 1 < functions_cnt)
> > +                       func->end = functions[i_func + 1].addr;
> > +
> > +               for (; i_addr < count; i_addr++) {
> > +                       __u64 addr = addrs[i_addr];
> > +
> > +                       /* Functions are  ahead, catch up with addrs. */
> > +                       if (addr < func->addr)
> > +                               continue;
> > +
> > +                       /* Addr is within function - mark function as valid. */
> > +                       if (is_addr_in_func(addr, func, kmod)) {
> > +                               /*
> > +                                * We iterate over sorted array, so we can easily skip
> > +                                * not valid item and move following valid field into
> > +                                * its place, and still keep the 'new' array sorted.
> > +                                */
> > +                               if (i_func != functions_valid)
> > +                                       functions[functions_valid] = functions[i_func];
> > +                               functions_valid++;
> > +                               i_addr++;
> > +                       }
> >
> > -               /* Make sure function is within ftrace addresses. */
> > -               if (bsearch(&addr, addrs, count, sizeof(addrs[0]), addrs_cmp)) {
> >                         /*
> > -                        * We iterate over sorted array, so we can easily skip
> > -                        * not valid item and move following valid field into
> > -                        * its place, and still keep the 'new' array sorted.
> > +                        * Addrs are ahead, catch up with functions, or we just
> > +                        * found valid function and want to move to another.
> >                          */
> > -                       if (i != functions_valid)
> > -                               functions[functions_valid] = functions[i];
> > -                       functions_valid++;
> > +                       break;
> >                 }
> >         }
> >
> > +       if (btf_elf__verbose) {
> > +               printf("Found %d functions out of %d symbols and %llu ftrace addresses.\n",
> > +                       functions_valid, functions_cnt, count);
> > +       }
> > +
> >         functions_cnt = functions_valid;
> >         free(addrs);
> >
> > -       if (btf_elf__verbose)
> > -               printf("Found %d functions!\n", functions_cnt);
> > +       /*
> > +        * And finaly sort 'valid' functions by name,
> > +        * so find_function can be used.
> > +        */
> > +       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)
> >
>