Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

Nathan Chancellor
2021-02-10T18:07:21Z

On Wed, Feb 10, 2021 at 02:26:43PM +0100, Jiri Olsa wrote:
> On Tue, Feb 09, 2021 at 02:00:29PM -0800, Andrii Nakryiko wrote:
>
> SNIP
>
> > > > > I'm still trying to build the kernel.. however ;-)
> > > > >
> > > > > patch below adds the ftrace check only for static functions
> > > > > and lets the externa go through.. but as you said, in this
> > > > > case we'll need to figure out the 'notrace' and other checks
> > > > > ftrace is doing
> > > > >
> > > > > jirka
> > > > >
> > > > >
> > > > > ---
> > > > > diff --git a/btf_encoder.c b/btf_encoder.c
> > > > > index b124ec20a689..4d147406cfa5 100644
> > > > > --- a/btf_encoder.c
> > > > > +++ b/btf_encoder.c
> > > > > @@ -734,7 +734,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
> > > > > continue;
> > > > > if (!has_arg_names(cu, &fn->proto))
> > > > > continue;
> > > > > - if (functions_cnt) {
> > > > > + if (!fn->external && functions_cnt) {
> > > >
> > > > I wouldn't trust DWARF, honestly. Wouldn't checking GLOBAL vs LOCAL
> > > > FUNC ELF symbol be more reliable?
> > >
> > > that'd mean extra bsearch on each processed function,
> > > on the ther hand, we'are already slow ;-) I'll check
> > > how big the slowdown would be
> > >
> >
> > We currently record addresses and do binary search. Now we need to
> > record address + size and still do binary search with a slightly
> > different semantics (find closest entry >= addr). Then just check that
> > it overlaps, taking into account the length of the function code. It
> > shouldn't result in a noticeable slowdown. Might be actually faster,
> > because we might avoid callback function call costs.
>
> I'm still not sure how to handle the external check for function via elf,
> 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
>
> jirka

This passes the build test with the initial problematic configuration
file and the reduced one that I came up with. That is about the extent
to which I can verify that it works though :)

> ---
> 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)
>