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Date: Fri, 12 Feb 2021 11:22:41 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jiri Olsa <jolsa@...hat.com>
Cc: Nathan Chancellor <nathan@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
clang-built-linux <clang-built-linux@...glegroups.com>,
Veronika Kabatova <vkabatov@...hat.com>,
Jiri Olsa <jolsa@...nel.org>
Subject: Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM
On Fri, Feb 12, 2021 at 8:39 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Thu, Feb 11, 2021 at 11:59:02AM -0800, Andrii Nakryiko wrote:
>
> SNIP
>
> >
> > 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);
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.
>
> /*
> * 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)
>
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