bpf: Add a bpf_snprintf helper

The implementation takes inspiration from the existing bpf_trace_printk helper but there are a few differences: To allow for a large number of format-specifiers, parameters are provided in an array, like in bpf_seq_printf. Because the output string takes two arguments and the array of parameters also takes two arguments, the format string needs to fit in one argument. Thankfully, ARG_PTR_TO_CONST_STR is guaranteed to point to a zero-terminated read-only map so we don't need a format string length arg. Because the format-string is known at verification time, we also do a first pass of format string validation in the verifier logic. This makes debugging easier. Signed-off-by: Florent Revest <[email protected]> Signed-off-by: Alexei Starovoitov <[email protected]> Acked-by: Andrii Nakryiko <[email protected]> Link: https://lore.kernel.org/bpf/[email protected]
torvalds · Apr 19, 2021 · 7b15523 · 7b15523
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commit 7b15523
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diff --git a/include/linux/bpf.h b/include/linux/bpf.h
@@ -1953,6 +1953,7 @@ extern const struct bpf_func_proto bpf_skc_to_tcp_request_sock_proto;
 extern const struct bpf_func_proto bpf_skc_to_udp6_sock_proto;
 extern const struct bpf_func_proto bpf_copy_from_user_proto;
 extern const struct bpf_func_proto bpf_snprintf_btf_proto;
+extern const struct bpf_func_proto bpf_snprintf_proto;
 extern const struct bpf_func_proto bpf_per_cpu_ptr_proto;
 extern const struct bpf_func_proto bpf_this_cpu_ptr_proto;
 extern const struct bpf_func_proto bpf_ktime_get_coarse_ns_proto;

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
@@ -4708,6 +4708,33 @@ union bpf_attr {
  * Return
  * The number of traversed map elements for success, **-EINVAL** for
  * invalid **flags**.
+ *
+ * long bpf_snprintf(char *str, u32 str_size, const char *fmt, u64 *data, u32 data_len)
+ * Description
+ * Outputs a string into the **str** buffer of size **str_size**
+ * based on a format string stored in a read-only map pointed by
+ * **fmt**.
+ *
+ * Each format specifier in **fmt** corresponds to one u64 element
+ * in the **data** array. For strings and pointers where pointees
+ * are accessed, only the pointer values are stored in the *data*
+ * array. The *data_len* is the size of *data* in bytes.
+ *
+ * Formats **%s** and **%p{i,I}{4,6}** require to read kernel
+ * memory. Reading kernel memory may fail due to either invalid
+ * address or valid address but requiring a major memory fault. If
+ * reading kernel memory fails, the string for **%s** will be an
+ * empty string, and the ip address for **%p{i,I}{4,6}** will be 0.
+ * Not returning error to bpf program is consistent with what
+ * **bpf_trace_printk**\ () does for now.
+ *
+ * Return
+ * The strictly positive length of the formatted string, including
+ * the trailing zero character. If the return value is greater than
+ * **str_size**, **str** contains a truncated string, guaranteed to
+ * be zero-terminated except when **str_size** is 0.
+ *
+ * Or **-EBUSY** if the per-CPU memory copy buffer is busy.
  */
 #define __BPF_FUNC_MAPPER(FN) \
  FN(unspec), \
@@ -4875,6 +4902,7 @@ union bpf_attr {
  FN(sock_from_file), \
  FN(check_mtu), \
  FN(for_each_map_elem), \
+ FN(snprintf), \
  /* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
@@ -925,6 +925,54 @@ int bpf_printf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
  return err;
 }
 
+#define MAX_SNPRINTF_VARARGS 12
+
+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;
+
+ if (data_len % 8 || data_len > MAX_SNPRINTF_VARARGS * 8 ||
+ (data_len && !data))
+ return -EINVAL;
+ num_args = data_len / 8;
+
+ /* 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);
+ 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();
+
+ return err + 1;
+}
+
+const struct bpf_func_proto bpf_snprintf_proto = {
+ .func = bpf_snprintf,
+ .gpl_only = true,
+ .ret_type = RET_INTEGER,
+ .arg1_type = ARG_PTR_TO_MEM_OR_NULL,
+ .arg2_type = ARG_CONST_SIZE_OR_ZERO,
+ .arg3_type = ARG_PTR_TO_CONST_STR,
+ .arg4_type = ARG_PTR_TO_MEM_OR_NULL,
+ .arg5_type = ARG_CONST_SIZE_OR_ZERO,
+};
+
 const struct bpf_func_proto bpf_get_current_task_proto __weak;
 const struct bpf_func_proto bpf_probe_read_user_proto __weak;
 const struct bpf_func_proto bpf_probe_read_user_str_proto __weak;
@@ -1013,6 +1061,8 @@ bpf_base_func_proto(enum bpf_func_id func_id)
  return &bpf_probe_read_kernel_str_proto;
  case BPF_FUNC_snprintf_btf:
  return &bpf_snprintf_btf_proto;
+ case BPF_FUNC_snprintf:
+ return &bpf_snprintf_proto;
  default:
  return NULL;
  }

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
@@ -5918,6 +5918,41 @@ static int check_reference_leak(struct bpf_verifier_env *env)
  return state->acquired_refs ? -EINVAL : 0;
 }
 
+static int check_bpf_snprintf_call(struct bpf_verifier_env *env,
+ struct bpf_reg_state *regs)
+{
+ struct bpf_reg_state *fmt_reg = &regs[BPF_REG_3];
+ struct bpf_reg_state *data_len_reg = &regs[BPF_REG_5];
+ struct bpf_map *fmt_map = fmt_reg->map_ptr;
+ int err, fmt_map_off, num_args;
+ u64 fmt_addr;
+ char *fmt;
+
+ /* data must be an array of u64 */
+ if (data_len_reg->var_off.value % 8)
+ return -EINVAL;
+ num_args = data_len_reg->var_off.value / 8;
+
+ /* fmt being ARG_PTR_TO_CONST_STR guarantees that var_off is const
+ * and map_direct_value_addr is set.
+ */
+ fmt_map_off = fmt_reg->off + fmt_reg->var_off.value;
+ err = fmt_map->ops->map_direct_value_addr(fmt_map, &fmt_addr,
+ fmt_map_off);
+ if (err)
+ return err;
+ fmt = (char *)(long)fmt_addr + fmt_map_off;
+
+ /* 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);
+ if (err < 0)
+ verbose(env, "Invalid format string\n");
+
+ return err;
+}
+
 static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
  int *insn_idx_p)
 {
@@ -6032,6 +6067,12 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
  return -EINVAL;
  }
 
+ if (func_id == BPF_FUNC_snprintf) {
+ err = check_bpf_snprintf_call(env, regs);
+ if (err < 0)
+ return err;
+ }
+
  /* reset caller saved regs */
  for (i = 0; i < CALLER_SAVED_REGS; i++) {
  mark_reg_not_init(env, regs, caller_saved[i]);

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
@@ -1076,6 +1076,8 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
  return &bpf_task_storage_delete_proto;
  case BPF_FUNC_for_each_map_elem:
  return &bpf_for_each_map_elem_proto;
+ case BPF_FUNC_snprintf:
+ return &bpf_snprintf_proto;
  default:
  return NULL;
  }

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
@@ -4708,6 +4708,33 @@ union bpf_attr {
  * Return
  * The number of traversed map elements for success, **-EINVAL** for
  * invalid **flags**.
+ *
+ * long bpf_snprintf(char *str, u32 str_size, const char *fmt, u64 *data, u32 data_len)
+ * Description
+ * Outputs a string into the **str** buffer of size **str_size**
+ * based on a format string stored in a read-only map pointed by
+ * **fmt**.
+ *
+ * Each format specifier in **fmt** corresponds to one u64 element
+ * in the **data** array. For strings and pointers where pointees
+ * are accessed, only the pointer values are stored in the *data*
+ * array. The *data_len* is the size of *data* in bytes.
+ *
+ * Formats **%s** and **%p{i,I}{4,6}** require to read kernel
+ * memory. Reading kernel memory may fail due to either invalid
+ * address or valid address but requiring a major memory fault. If
+ * reading kernel memory fails, the string for **%s** will be an
+ * empty string, and the ip address for **%p{i,I}{4,6}** will be 0.
+ * Not returning error to bpf program is consistent with what
+ * **bpf_trace_printk**\ () does for now.
+ *
+ * Return
+ * The strictly positive length of the formatted string, including
+ * the trailing zero character. If the return value is greater than
+ * **str_size**, **str** contains a truncated string, guaranteed to
+ * be zero-terminated except when **str_size** is 0.
+ *
+ * Or **-EBUSY** if the per-CPU memory copy buffer is busy.
  */
 #define __BPF_FUNC_MAPPER(FN) \
  FN(unspec), \
@@ -4875,6 +4902,7 @@ union bpf_attr {
  FN(sock_from_file), \
  FN(check_mtu), \
  FN(for_each_map_elem), \
+ FN(snprintf), \
  /* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper