Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

Nathan Chancellor
2021-02-11T17:27:08Z

On Thu, Feb 11, 2021 at 04:08:11PM +0100, Jiri Olsa wrote:
> On Wed, Feb 10, 2021 at 09:13:47PM +0100, Jiri Olsa wrote:
> > 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?
>
> heya,
> with your approach I ended up with change below, it gives me same
> results as with the previous change
>
> I think I'll separate the kmod bool address computation later on,
> but I did not want to confuse this change for now
>
> jirka

This still looks good against the original and reduced problematic
configurations for aarch64.

Tested-by: Nathan Chancellor <nathan@kernel.org>

> ---
> diff --git a/btf_encoder.c b/btf_encoder.c
> index b124ec20a689..34df08f2fb4e 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++;
> @@ -236,6 +248,40 @@ 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 end = kmod ? func->end + func->sh_addr : func->end;
> +
> + size_t l = 0, r = count - 1, m;
> + __u64 addr = 0;
> +
> + while (l < r) {
> + m = l + (r - l + 1) / 2;
> + addr = addrs[m];
> +
> + if (start <= addr && addr < end)
> + return true;
> +
> + if (start <= addr)
> + r = m - 1;
> + else
> + l = m;
> + }
> +
> + addr = addrs[l];
> + return start <= addr && addr < end;
> +}
> +
> static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
> {
> __u64 *addrs, count, i;
> @@ -267,7 +313,7 @@ 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 +321,12 @@ 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;
> +
> + if (i + 1 < functions_cnt)
> + func->end = functions[i + 1].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 +343,8 @@ 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 +354,7 @@ 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)
>