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bpf: Implement formatted output helpers with bstr_printf
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BPF has three formatted output helpers: bpf_trace_printk, bpf_seq_printf
and bpf_snprintf. Their signatures specify that all arguments are
provided from the BPF world as u64s (in an array or as registers). All
of these helpers are currently implemented by calling functions such as
snprintf() whose signatures take a variable number of arguments, then
placed in a va_list by the compiler to call vsnprintf().

"d9c9e4db bpf: Factorize bpf_trace_printk and bpf_seq_printf" introduced
a bpf_printf_prepare function that fills an array of u64 sanitized
arguments with an array of "modifiers" which indicate what the "real"
size of each argument should be (given by the format specifier). The
BPF_CAST_FMT_ARG macro consumes these arrays and casts each argument to
its real size. However, the C promotion rules implicitely cast them all
back to u64s. Therefore, the arguments given to snprintf are u64s and
the va_list constructed by the compiler will use 64 bits for each
argument. On 64 bit machines, this happens to work well because 32 bit
arguments in va_lists need to occupy 64 bits anyway, but on 32 bit
architectures this breaks the layout of the va_list expected by the
called function and mangles values.

In "88a5c690b6 bpf: fix bpf_trace_printk on 32 bit archs", this problem
had been solved for bpf_trace_printk only with a "horrid workaround"
that emitted multiple calls to trace_printk where each call had
different argument types and generated different va_list layouts. One of
the call would be dynamically chosen at runtime. This was ok with the 3
arguments that bpf_trace_printk takes but bpf_seq_printf and
bpf_snprintf accept up to 12 arguments. Because this approach scales
code exponentially, it is not a viable option anymore.

Because the promotion rules are part of the language and because the
construction of a va_list is an arch-specific ABI, it's best to just
avoid variadic arguments and va_lists altogether. Thankfully the
kernel's snprintf() has an alternative in the form of bstr_printf() that
accepts arguments in a "binary buffer representation". These binary
buffers are currently created by vbin_printf and used in the tracing
subsystem to split the cost of printing into two parts: a fast one that
only dereferences and remembers values, and a slower one, called later,
that does the pretty-printing.

This patch refactors bpf_printf_prepare to construct binary buffers of
arguments consumable by bstr_printf() instead of arrays of arguments and
modifiers. This gets rid of BPF_CAST_FMT_ARG and greatly simplifies the
bpf_printf_prepare usage but there are a few gotchas that change how
bpf_printf_prepare needs to do things.

Currently, bpf_printf_prepare uses a per cpu temporary buffer as a
generic storage for strings and IP addresses. With this refactoring, the
temporary buffers now holds all the arguments in a structured binary
format.

To comply with the format expected by bstr_printf, certain format
specifiers also need to be pre-formatted: %pB and %pi6/%pi4/%pI4/%pI6.
Because vsnprintf subroutines for these specifiers are hard to expose,
we pre-format these arguments with calls to snprintf().

Reported-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Florent Revest <revest@chromium.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210427174313.860948-3-revest@chromium.org
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@FlorentRevest
FlorentRevest authored and Alexei Starovoitov committed Apr 27, 2021
1 parent 76d6a13 commit 48cac3f
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Showing 5 changed files with 115 additions and 132 deletions.
22 changes: 3 additions & 19 deletions include/linux/bpf.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2081,24 +2081,8 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
struct btf_id_set;
bool btf_id_set_contains(const struct btf_id_set *set, u32 id);

enum bpf_printf_mod_type {
BPF_PRINTF_INT,
BPF_PRINTF_LONG,
BPF_PRINTF_LONG_LONG,
};

/* Workaround for getting va_list handling working with different argument type
* combinations generically for 32 and 64 bit archs.
*/
#define BPF_CAST_FMT_ARG(arg_nb, args, mod) \
(mod[arg_nb] == BPF_PRINTF_LONG_LONG || \
(mod[arg_nb] == BPF_PRINTF_LONG && __BITS_PER_LONG == 64) \
? (u64)args[arg_nb] \
: (u32)args[arg_nb])

int bpf_printf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
u64 *final_args, enum bpf_printf_mod_type *mod,
u32 num_args);
void bpf_printf_cleanup(void);
int bpf_bprintf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
u32 **bin_buf, u32 num_args);
void bpf_bprintf_cleanup(void);

#endif /* _LINUX_BPF_H */
1 change: 1 addition & 0 deletions init/Kconfig
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1708,6 +1708,7 @@ config BPF_SYSCALL
select BPF
select IRQ_WORK
select TASKS_TRACE_RCU
select BINARY_PRINTF
select NET_SOCK_MSG if INET
default n
help
Expand Down
188 changes: 100 additions & 88 deletions kernel/bpf/helpers.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -707,9 +707,6 @@ static int try_get_fmt_tmp_buf(char **tmp_buf)
struct bpf_printf_buf *bufs;
int used;

if (*tmp_buf)
return 0;

preempt_disable();
used = this_cpu_inc_return(bpf_printf_buf_used);
if (WARN_ON_ONCE(used > 1)) {
Expand All @@ -723,7 +720,7 @@ static int try_get_fmt_tmp_buf(char **tmp_buf)
return 0;
}

void bpf_printf_cleanup(void)
void bpf_bprintf_cleanup(void)
{
if (this_cpu_read(bpf_printf_buf_used)) {
this_cpu_dec(bpf_printf_buf_used);
Expand All @@ -732,43 +729,45 @@ void bpf_printf_cleanup(void)
}

/*
* bpf_parse_fmt_str - Generic pass on format strings for printf-like helpers
* bpf_bprintf_prepare - Generic pass on format strings for bprintf-like helpers
*
* Returns a negative value if fmt is an invalid format string or 0 otherwise.
*
* This can be used in two ways:
* - Format string verification only: when final_args and mod are NULL
* - Format string verification only: when bin_args is NULL
* - Arguments preparation: in addition to the above verification, it writes in
* final_args a copy of raw_args where pointers from BPF have been sanitized
* into pointers safe to use by snprintf. This also writes in the mod array
* the size requirement of each argument, usable by BPF_CAST_FMT_ARG for ex.
* bin_args a binary representation of arguments usable by bstr_printf where
* pointers from BPF have been sanitized.
*
* In argument preparation mode, if 0 is returned, safe temporary buffers are
* allocated and bpf_printf_cleanup should be called to free them after use.
* allocated and bpf_bprintf_cleanup should be called to free them after use.
*/
int bpf_printf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
u64 *final_args, enum bpf_printf_mod_type *mod,
u32 num_args)
int bpf_bprintf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
u32 **bin_args, u32 num_args)
{
char *unsafe_ptr = NULL, *tmp_buf = NULL, *fmt_end;
size_t tmp_buf_len = MAX_PRINTF_BUF_LEN;
int err, i, num_spec = 0, copy_size;
enum bpf_printf_mod_type cur_mod;
char *unsafe_ptr = NULL, *tmp_buf = NULL, *tmp_buf_end, *fmt_end;
size_t sizeof_cur_arg, sizeof_cur_ip;
int err, i, num_spec = 0;
u64 cur_arg;
char fmt_ptype;

if (!!final_args != !!mod)
return -EINVAL;
char fmt_ptype, cur_ip[16], ip_spec[] = "%pXX";

fmt_end = strnchr(fmt, fmt_size, 0);
if (!fmt_end)
return -EINVAL;
fmt_size = fmt_end - fmt;

if (bin_args) {
if (num_args && try_get_fmt_tmp_buf(&tmp_buf))
return -EBUSY;

tmp_buf_end = tmp_buf + MAX_PRINTF_BUF_LEN;
*bin_args = (u32 *)tmp_buf;
}

for (i = 0; i < fmt_size; i++) {
if ((!isprint(fmt[i]) && !isspace(fmt[i])) || !isascii(fmt[i])) {
err = -EINVAL;
goto cleanup;
goto out;
}

if (fmt[i] != '%')
Expand All @@ -781,7 +780,7 @@ int bpf_printf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,

if (num_spec >= num_args) {
err = -EINVAL;
goto cleanup;
goto out;
}

/* The string is zero-terminated so if fmt[i] != 0, we can
Expand All @@ -800,7 +799,7 @@ int bpf_printf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
}

if (fmt[i] == 'p') {
cur_mod = BPF_PRINTF_LONG;
sizeof_cur_arg = sizeof(long);

if ((fmt[i + 1] == 'k' || fmt[i + 1] == 'u') &&
fmt[i + 2] == 's') {
Expand All @@ -811,117 +810,140 @@ int bpf_printf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,

if (fmt[i + 1] == 0 || isspace(fmt[i + 1]) ||
ispunct(fmt[i + 1]) || fmt[i + 1] == 'K' ||
fmt[i + 1] == 'x' || fmt[i + 1] == 'B' ||
fmt[i + 1] == 's' || fmt[i + 1] == 'S') {
fmt[i + 1] == 'x' || fmt[i + 1] == 's' ||
fmt[i + 1] == 'S') {
/* just kernel pointers */
if (final_args)
if (tmp_buf)
cur_arg = raw_args[num_spec];
goto fmt_next;
i++;
goto nocopy_fmt;
}

if (fmt[i + 1] == 'B') {
if (tmp_buf) {
err = snprintf(tmp_buf,
(tmp_buf_end - tmp_buf),
"%pB",
(void *)(long)raw_args[num_spec]);
tmp_buf += (err + 1);
}

i++;
num_spec++;
continue;
}

/* only support "%pI4", "%pi4", "%pI6" and "%pi6". */
if ((fmt[i + 1] != 'i' && fmt[i + 1] != 'I') ||
(fmt[i + 2] != '4' && fmt[i + 2] != '6')) {
err = -EINVAL;
goto cleanup;
goto out;
}

i += 2;
if (!final_args)
goto fmt_next;
if (!tmp_buf)
goto nocopy_fmt;

if (try_get_fmt_tmp_buf(&tmp_buf)) {
err = -EBUSY;
goto out;
}

copy_size = (fmt[i + 2] == '4') ? 4 : 16;
if (tmp_buf_len < copy_size) {
sizeof_cur_ip = (fmt[i] == '4') ? 4 : 16;
if (tmp_buf_end - tmp_buf < sizeof_cur_ip) {
err = -ENOSPC;
goto cleanup;
goto out;
}

unsafe_ptr = (char *)(long)raw_args[num_spec];
err = copy_from_kernel_nofault(tmp_buf, unsafe_ptr,
copy_size);
err = copy_from_kernel_nofault(cur_ip, unsafe_ptr,
sizeof_cur_ip);
if (err < 0)
memset(tmp_buf, 0, copy_size);
cur_arg = (u64)(long)tmp_buf;
tmp_buf += copy_size;
tmp_buf_len -= copy_size;
memset(cur_ip, 0, sizeof_cur_ip);

/* hack: bstr_printf expects IP addresses to be
* pre-formatted as strings, ironically, the easiest way
* to do that is to call snprintf.
*/
ip_spec[2] = fmt[i - 1];
ip_spec[3] = fmt[i];
err = snprintf(tmp_buf, tmp_buf_end - tmp_buf,
ip_spec, &cur_ip);

goto fmt_next;
tmp_buf += err + 1;
num_spec++;

continue;
} else if (fmt[i] == 's') {
cur_mod = BPF_PRINTF_LONG;
fmt_ptype = fmt[i];
fmt_str:
if (fmt[i + 1] != 0 &&
!isspace(fmt[i + 1]) &&
!ispunct(fmt[i + 1])) {
err = -EINVAL;
goto cleanup;
}

if (!final_args)
goto fmt_next;

if (try_get_fmt_tmp_buf(&tmp_buf)) {
err = -EBUSY;
goto out;
}

if (!tmp_buf_len) {
if (!tmp_buf)
goto nocopy_fmt;

if (tmp_buf_end == tmp_buf) {
err = -ENOSPC;
goto cleanup;
goto out;
}

unsafe_ptr = (char *)(long)raw_args[num_spec];
err = bpf_trace_copy_string(tmp_buf, unsafe_ptr,
fmt_ptype, tmp_buf_len);
fmt_ptype,
tmp_buf_end - tmp_buf);
if (err < 0) {
tmp_buf[0] = '\0';
err = 1;
}

cur_arg = (u64)(long)tmp_buf;
tmp_buf += err;
tmp_buf_len -= err;
num_spec++;

goto fmt_next;
continue;
}

cur_mod = BPF_PRINTF_INT;
sizeof_cur_arg = sizeof(int);

if (fmt[i] == 'l') {
cur_mod = BPF_PRINTF_LONG;
sizeof_cur_arg = sizeof(long);
i++;
}
if (fmt[i] == 'l') {
cur_mod = BPF_PRINTF_LONG_LONG;
sizeof_cur_arg = sizeof(long long);
i++;
}

if (fmt[i] != 'i' && fmt[i] != 'd' && fmt[i] != 'u' &&
fmt[i] != 'x' && fmt[i] != 'X') {
err = -EINVAL;
goto cleanup;
goto out;
}

if (final_args)
if (tmp_buf)
cur_arg = raw_args[num_spec];
fmt_next:
if (final_args) {
mod[num_spec] = cur_mod;
final_args[num_spec] = cur_arg;
nocopy_fmt:
if (tmp_buf) {
tmp_buf = PTR_ALIGN(tmp_buf, sizeof(u32));
if (tmp_buf_end - tmp_buf < sizeof_cur_arg) {
err = -ENOSPC;
goto out;
}

if (sizeof_cur_arg == 8) {
*(u32 *)tmp_buf = *(u32 *)&cur_arg;
*(u32 *)(tmp_buf + 4) = *((u32 *)&cur_arg + 1);
} else {
*(u32 *)tmp_buf = (u32)(long)cur_arg;
}
tmp_buf += sizeof_cur_arg;
}
num_spec++;
}

err = 0;
cleanup:
if (err)
bpf_printf_cleanup();
out:
if (err)
bpf_bprintf_cleanup();
return err;
}

Expand All @@ -930,9 +952,8 @@ int bpf_printf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
BPF_CALL_5(bpf_snprintf, char *, str, u32, str_size, char *, fmt,
const void *, data, u32, data_len)
{
enum bpf_printf_mod_type mod[MAX_SNPRINTF_VARARGS];
u64 args[MAX_SNPRINTF_VARARGS];
int err, num_args;
u32 *bin_args;

if (data_len % 8 || data_len > MAX_SNPRINTF_VARARGS * 8 ||
(data_len && !data))
Expand All @@ -942,22 +963,13 @@ BPF_CALL_5(bpf_snprintf, char *, str, u32, str_size, char *, fmt,
/* ARG_PTR_TO_CONST_STR guarantees that fmt is zero-terminated so we
* can safely give an unbounded size.
*/
err = bpf_printf_prepare(fmt, UINT_MAX, data, args, mod, num_args);
err = bpf_bprintf_prepare(fmt, UINT_MAX, data, &bin_args, num_args);
if (err < 0)
return err;

/* Maximumly we can have MAX_SNPRINTF_VARARGS parameters, just give
* all of them to snprintf().
*/
err = snprintf(str, str_size, fmt, BPF_CAST_FMT_ARG(0, args, mod),
BPF_CAST_FMT_ARG(1, args, mod), BPF_CAST_FMT_ARG(2, args, mod),
BPF_CAST_FMT_ARG(3, args, mod), BPF_CAST_FMT_ARG(4, args, mod),
BPF_CAST_FMT_ARG(5, args, mod), BPF_CAST_FMT_ARG(6, args, mod),
BPF_CAST_FMT_ARG(7, args, mod), BPF_CAST_FMT_ARG(8, args, mod),
BPF_CAST_FMT_ARG(9, args, mod), BPF_CAST_FMT_ARG(10, args, mod),
BPF_CAST_FMT_ARG(11, args, mod));

bpf_printf_cleanup();
err = bstr_printf(str, str_size, fmt, bin_args);

bpf_bprintf_cleanup();

return err + 1;
}
Expand Down
2 changes: 1 addition & 1 deletion kernel/bpf/verifier.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -5946,7 +5946,7 @@ static int check_bpf_snprintf_call(struct bpf_verifier_env *env,
/* We are also guaranteed that fmt+fmt_map_off is NULL terminated, we
* can focus on validating the format specifiers.
*/
err = bpf_printf_prepare(fmt, UINT_MAX, NULL, NULL, NULL, num_args);
err = bpf_bprintf_prepare(fmt, UINT_MAX, NULL, NULL, num_args);
if (err < 0)
verbose(env, "Invalid format string\n");

Expand Down
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