On Thu, May 06, 2021 at 01:56:22PM -0700, Martin KaFai Lau wrote:
> BTF is currently generated for functions that are in ftrace list
> or extern.
>
> A recent use case also needs BTF generated for functions included in
> allowlist. In particular, the kernel
> commit e78aea8b2170 ("bpf: tcp: Put some tcp cong functions in allowlist for bpf-tcp-cc")
> allows bpf program to directly call a few tcp cc kernel functions. Those
> kernel functions are currently allowed only if CONFIG_DYNAMIC_FTRACE
> is set to ensure they are in the ftrace list but this kconfig dependency
> is unnecessary.
>
> Those kernel functions are specified under an ELF section .BTF_ids.
> There was an earlier attempt [0] to add another filter for the functions in
> the .BTF_ids section. That discussion concluded that the ftrace filter
> should be removed instead.
>
> This patch is to remove the ftrace filter and its related functions.
>
> Number of BTF FUNC with and without is_ftrace_func():
> My kconfig in x86: 40643 vs 46225
> Jiri reported on arm: 25022 vs 55812
>
> [0]: https://lore.kernel.org/dwarves/20210423213728.3538141-1-kafai@xxxxxx/
>
> Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Signed-off-by: Martin KaFai Lau <kafai@xxxxxx>
This fixes an issue with Fedora's s390x config that CKI noticed:
https://groups.google.com/g/clang-built-linux/c/IzthpckBJvc/m/MPWGDmXiAwAJ
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx> # build
> ---
> v2: Remove sym_sec_idx, last_idx, and sh. (Jiri Olsa)
>
> btf_encoder.c | 285 ++------------------------------------------------
> 1 file changed, 7 insertions(+), 278 deletions(-)
>
> diff --git a/btf_encoder.c b/btf_encoder.c
> index 80e896961d4e..c711f124b31e 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -27,17 +27,8 @@
> */
> #define KSYM_NAME_LEN 128
>
> -struct funcs_layout {
> - unsigned long mcount_start;
> - unsigned long mcount_stop;
> - unsigned long mcount_sec_idx;
> -};
> -
> struct elf_function {
> const char *name;
> - unsigned long addr;
> - unsigned long size;
> - unsigned long sh_addr;
> bool generated;
> };
>
> @@ -64,12 +55,9 @@ static void delete_functions(void)
> #define max(x, y) ((x) < (y) ? (y) : (x))
> #endif
>
> -static int collect_function(struct btf_elf *btfe, GElf_Sym *sym,
> - size_t sym_sec_idx)
> +static int collect_function(struct btf_elf *btfe, GElf_Sym *sym)
> {
> struct elf_function *new;
> - static GElf_Shdr sh;
> - static size_t last_idx;
> const char *name;
>
> if (elf_sym__type(sym) != STT_FUNC)
> @@ -91,257 +79,12 @@ static int collect_function(struct btf_elf *btfe, GElf_Sym *sym,
> functions = new;
> }
>
> - if (sym_sec_idx != last_idx) {
> - if (!elf_section_by_idx(btfe->elf, &sh, sym_sec_idx))
> - return 0;
> - last_idx = sym_sec_idx;
> - }
> -
> 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++;
> return 0;
> }
>
> -static int addrs_cmp(const void *_a, const void *_b)
> -{
> - const __u64 *a = _a;
> - const __u64 *b = _b;
> -
> - if (*a == *b)
> - return 0;
> - return *a < *b ? -1 : 1;
> -}
> -
> -static int get_vmlinux_addrs(struct btf_elf *btfe, struct funcs_layout *fl,
> - __u64 **paddrs, __u64 *pcount)
> -{
> - __u64 *addrs, count, offset;
> - unsigned int addr_size, i;
> - Elf_Data *data;
> - GElf_Shdr shdr;
> - Elf_Scn *sec;
> -
> - /* Initialize for the sake of all error paths below. */
> - *paddrs = NULL;
> - *pcount = 0;
> -
> - if (!fl->mcount_start || !fl->mcount_stop)
> - return 0;
> -
> - /*
> - * Find mcount addressed marked by __start_mcount_loc
> - * and __stop_mcount_loc symbols and load them into
> - * sorted array.
> - */
> - sec = elf_getscn(btfe->elf, fl->mcount_sec_idx);
> - if (!sec || !gelf_getshdr(sec, &shdr)) {
> - fprintf(stderr, "Failed to get section(%lu) header.\n",
> - fl->mcount_sec_idx);
> - return -1;
> - }
> -
> - /* Get address size from processed file's ELF class. */
> - addr_size = gelf_getclass(btfe->elf) == ELFCLASS32 ? 4 : 8;
> -
> - offset = fl->mcount_start - shdr.sh_addr;
> - count = (fl->mcount_stop - fl->mcount_start) / addr_size;
> -
> - data = elf_getdata(sec, 0);
> - if (!data) {
> - fprintf(stderr, "Failed to get section(%lu) data.\n",
> - fl->mcount_sec_idx);
> - return -1;
> - }
> -
> - addrs = malloc(count * sizeof(addrs[0]));
> - if (!addrs) {
> - fprintf(stderr, "Failed to allocate memory for ftrace addresses.\n");
> - return -1;
> - }
> -
> - if (addr_size == sizeof(__u64)) {
> - memcpy(addrs, data->d_buf + offset, count * addr_size);
> - } else {
> - for (i = 0; i < count; i++)
> - addrs[i] = (__u64) *((__u32 *) (data->d_buf + offset + i * addr_size));
> - }
> -
> - *paddrs = addrs;
> - *pcount = count;
> - return 0;
> -}
> -
> -static int
> -get_kmod_addrs(struct btf_elf *btfe, __u64 **paddrs, __u64 *pcount)
> -{
> - __u64 *addrs, count;
> - unsigned int addr_size, i;
> - GElf_Shdr shdr_mcount;
> - Elf_Data *data;
> - Elf_Scn *sec;
> -
> - /* Initialize for the sake of all error paths below. */
> - *paddrs = NULL;
> - *pcount = 0;
> -
> - /* get __mcount_loc */
> - sec = elf_section_by_name(btfe->elf, &btfe->ehdr, &shdr_mcount,
> - "__mcount_loc", NULL);
> - if (!sec) {
> - if (btf_elf__verbose) {
> - printf("%s: '%s' doesn't have __mcount_loc section\n", __func__,
> - btfe->filename);
> - }
> - return 0;
> - }
> -
> - data = elf_getdata(sec, NULL);
> - if (!data) {
> - fprintf(stderr, "Failed to data for __mcount_loc section.\n");
> - return -1;
> - }
> -
> - /* Get address size from processed file's ELF class. */
> - addr_size = gelf_getclass(btfe->elf) == ELFCLASS32 ? 4 : 8;
> -
> - count = data->d_size / addr_size;
> -
> - addrs = malloc(count * sizeof(addrs[0]));
> - if (!addrs) {
> - fprintf(stderr, "Failed to allocate memory for ftrace addresses.\n");
> - return -1;
> - }
> -
> - if (addr_size == sizeof(__u64)) {
> - memcpy(addrs, data->d_buf, count * addr_size);
> - } else {
> - for (i = 0; i < count; i++)
> - addrs[i] = (__u64) *((__u32 *) (data->d_buf + i * addr_size));
> - }
> -
> - /*
> - * We get Elf object from dwfl_module_getelf function,
> - * which performs all possible relocations, including
> - * __mcount_loc section.
> - *
> - * So addrs array now contains relocated values, which
> - * we need take into account when we compare them to
> - * functions values, see comment in setup_functions
> - * function.
> - */
> - *paddrs = addrs;
> - *pcount = count;
> - return 0;
> -}
> -
> -static int is_ftrace_func(struct elf_function *func, __u64 *addrs, __u64 count)
> -{
> - __u64 start = 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;
> - int functions_valid = 0;
> - bool kmod = false;
> -
> - /*
> - * Check if we are processing vmlinux image and
> - * get mcount data if it's detected.
> - */
> - if (get_vmlinux_addrs(btfe, fl, &addrs, &count))
> - return -1;
> -
> - /*
> - * Check if we are processing kernel module and
> - * get mcount data if it's detected.
> - */
> - if (!addrs) {
> - if (get_kmod_addrs(btfe, &addrs, &count))
> - return -1;
> - kmod = true;
> - }
> -
> - if (!addrs) {
> - if (btf_elf__verbose)
> - printf("ftrace symbols not detected, falling back to DWARF data\n");
> - delete_functions();
> - 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.
> - */
> - 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.
> - */
> - if (kmod)
> - func->addr += func->sh_addr;
> -
> - /* Make sure function is within ftrace addresses. */
> - if (is_ftrace_func(func, addrs, count)) {
> - /*
> - * 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 != functions_valid)
> - functions[functions_valid] = functions[i];
> - functions_valid++;
> - }
> - }
> -
> - functions_cnt = functions_valid;
> - free(addrs);
> -
> - if (btf_elf__verbose)
> - printf("Found %d functions!\n", functions_cnt);
> - return 0;
> -}
> -
> static struct elf_function *find_function(const struct btf_elf *btfe,
> const char *name)
> {
> @@ -620,23 +363,8 @@ static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym,
> return 0;
> }
>
> -static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl,
> - size_t sym_sec_idx)
> -{
> - if (!fl->mcount_start &&
> - !strcmp("__start_mcount_loc", elf_sym__name(sym, btfe->symtab))) {
> - fl->mcount_start = sym->st_value;
> - fl->mcount_sec_idx = sym_sec_idx;
> - }
> -
> - if (!fl->mcount_stop &&
> - !strcmp("__stop_mcount_loc", elf_sym__name(sym, btfe->symtab)))
> - fl->mcount_stop = sym->st_value;
> -}
> -
> static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
> {
> - struct funcs_layout fl = { };
> Elf32_Word sym_sec_idx;
> uint32_t core_id;
> GElf_Sym sym;
> @@ -648,9 +376,8 @@ static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
> elf_symtab__for_each_symbol_index(btfe->symtab, core_id, sym, sym_sec_idx) {
> if (collect_percpu_vars && collect_percpu_var(btfe, &sym, sym_sec_idx))
> return -1;
> - if (collect_function(btfe, &sym, sym_sec_idx))
> + if (collect_function(btfe, &sym))
> return -1;
> - collect_symbol(&sym, &fl, sym_sec_idx);
> }
>
> if (collect_percpu_vars) {
> @@ -661,9 +388,11 @@ static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
> printf("Found %d per-CPU variables!\n", percpu_var_cnt);
> }
>
> - if (functions_cnt && setup_functions(btfe, &fl)) {
> - fprintf(stderr, "Failed to filter DWARF functions\n");
> - return -1;
> + if (functions_cnt) {
> + qsort(functions, functions_cnt, sizeof(functions[0]),
> + functions_cmp);
> + if (btf_elf__verbose)
> + printf("Found %d functions!\n", functions_cnt);
> }
>
> return 0;
> --
> 2.30.2
>
