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btf: Remove ftrace filter
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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@fb.com/

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Tested-by: Nathan Chancellor <nathan@kernel.org> # build
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Jiri Slaby <jirislaby@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: kernel-team@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
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@iamkafai @acmel
iamkafai authored and acmel committed May 8, 2021
1 parent 7c60b04 commit 58a98f7
Showing 1 changed file with 7 additions and 278 deletions.
285 changes: 7 additions & 278 deletions btf_encoder.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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;
};

Expand All @@ -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)
Expand All @@ -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)
{
Expand Down Expand Up @@ -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;
Expand All @@ -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) {
Expand All @@ -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;
Expand Down

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