Merge branch 'Add a snprintf eBPF helper'

Florent Revest says:

====================

We have a usecase where we want to audit symbol names (if available) in
callback registration hooks. (ex: fentry/nf_register_net_hook)

A few months back, I proposed a bpf_kallsyms_lookup series but it was
decided in the reviews that a more generic helper, bpf_snprintf, would
be more useful.

This series implements the helper according to the feedback received in
https://lore.kernel.org/bpf/20201126165748.1748417-1-revest@google.com/T/#u

- A new arg type guarantees the NULL-termination of string arguments and
  lets us pass format strings in only one arg
- A new helper is implemented using that guarantee. Because the format
  string is known at verification time, the format string validation is
  done by the verifier
- To implement a series of tests for bpf_snprintf, the logic for
  marshalling variadic args in a fixed-size array is reworked as per:
https://lore.kernel.org/bpf/20210310015455.1095207-1-revest@chromium.org/T/#u

---
Changes in v5:
- Fixed the bpf_printf_buf_used counter logic in try_get_fmt_tmp_buf
- Added a couple of extra incorrect specifiers tests
- Call test_snprintf_single__destroy unconditionally
- Fixed a C++-style comment

---
Changes in v4:
- Moved bpf_snprintf, bpf_printf_prepare and bpf_printf_cleanup to
  kernel/bpf/helpers.c so that they get built without CONFIG_BPF_EVENTS
- Added negative test cases (various invalid format strings)
- Renamed put_fmt_tmp_buf() as bpf_printf_cleanup()
- Fixed a mistake that caused temporary buffers to be unconditionally
  freed in bpf_printf_prepare
- Fixed a mistake that caused missing 0 character to be ignored
- Fixed a warning about integer to pointer conversion
- Misc cleanups

---
Changes in v3:
- Simplified temporary buffer acquisition with try_get_fmt_tmp_buf()
- Made zero-termination check more consistent
- Allowed NULL output_buffer
- Simplified the BPF_CAST_FMT_ARG macro
- Three new test cases: number padding, simple string with no arg and
  string length extraction only with a NULL output buffer
- Clarified helper's description for edge cases (eg: str_size == 0)
- Lots of cosmetic changes

---
Changes in v2:
- Extracted the format validation/argument sanitization in a generic way
  for all printf-like helpers.
- bpf_snprintf's str_size can now be 0
- bpf_snprintf is now exposed to all BPF program types
- We now preempt_disable when using a per-cpu temporary buffer
- Addressed a few cosmetic changes
====================

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 47 insertions, 11 deletions

diff --git a/tools/lib/bpf/bpf_tracing.h b/tools/lib/bpf/bpf_tracing.h
index f9ef37707888..8c954ebc0c7c 100644
--- a/tools/lib/bpf/bpf_tracing.h
+++ b/tools/lib/bpf/bpf_tracing.h
@@ -413,20 +413,56 @@ typeof(name(0)) name(struct pt_regs *ctx)				    \
 }									    \
 static __always_inline typeof(name(0)) ____##name(struct pt_regs *ctx, ##args)
 
+#define ___bpf_fill0(arr, p, x) do {} while (0)
+#define ___bpf_fill1(arr, p, x) arr[p] = x
+#define ___bpf_fill2(arr, p, x, args...) arr[p] = x; ___bpf_fill1(arr, p + 1, args)
+#define ___bpf_fill3(arr, p, x, args...) arr[p] = x; ___bpf_fill2(arr, p + 1, args)
+#define ___bpf_fill4(arr, p, x, args...) arr[p] = x; ___bpf_fill3(arr, p + 1, args)
+#define ___bpf_fill5(arr, p, x, args...) arr[p] = x; ___bpf_fill4(arr, p + 1, args)
+#define ___bpf_fill6(arr, p, x, args...) arr[p] = x; ___bpf_fill5(arr, p + 1, args)
+#define ___bpf_fill7(arr, p, x, args...) arr[p] = x; ___bpf_fill6(arr, p + 1, args)
+#define ___bpf_fill8(arr, p, x, args...) arr[p] = x; ___bpf_fill7(arr, p + 1, args)
+#define ___bpf_fill9(arr, p, x, args...) arr[p] = x; ___bpf_fill8(arr, p + 1, args)
+#define ___bpf_fill10(arr, p, x, args...) arr[p] = x; ___bpf_fill9(arr, p + 1, args)
+#define ___bpf_fill11(arr, p, x, args...) arr[p] = x; ___bpf_fill10(arr, p + 1, args)
+#define ___bpf_fill12(arr, p, x, args...) arr[p] = x; ___bpf_fill11(arr, p + 1, args)
+#define ___bpf_fill(arr, args...) \
+	___bpf_apply(___bpf_fill, ___bpf_narg(args))(arr, 0, args)
+
 /*
  * BPF_SEQ_PRINTF to wrap bpf_seq_printf to-be-printed values
  * in a structure.
  */
-#define BPF_SEQ_PRINTF(seq, fmt, args...)				    \
-	({								    \
-		_Pragma("GCC diagnostic push")				    \
-		_Pragma("GCC diagnostic ignored \"-Wint-conversion\"")	    \
-		static const char ___fmt[] = fmt;			    \
-		unsigned long long ___param[] = { args };		    \
-		_Pragma("GCC diagnostic pop")				    \
-		int ___ret = bpf_seq_printf(seq, ___fmt, sizeof(___fmt),    \
-					    ___param, sizeof(___param));    \
-		___ret;							    \
-	})
+#define BPF_SEQ_PRINTF(seq, fmt, args...)			\
+({								\
+	static const char ___fmt[] = fmt;			\
+	unsigned long long ___param[___bpf_narg(args)];		\
+								\
+	_Pragma("GCC diagnostic push")				\
+	_Pragma("GCC diagnostic ignored \"-Wint-conversion\"")	\
+	___bpf_fill(___param, args);				\
+	_Pragma("GCC diagnostic pop")				\
+								\
+	bpf_seq_printf(seq, ___fmt, sizeof(___fmt),		\
+		       ___param, sizeof(___param));		\
+})
+
+/*
+ * BPF_SNPRINTF wraps the bpf_snprintf helper with variadic arguments instead of
+ * an array of u64.
+ */
+#define BPF_SNPRINTF(out, out_size, fmt, args...)		\
+({								\
+	static const char ___fmt[] = fmt;			\
+	unsigned long long ___param[___bpf_narg(args)];		\
+								\
+	_Pragma("GCC diagnostic push")				\
+	_Pragma("GCC diagnostic ignored \"-Wint-conversion\"")	\
+	___bpf_fill(___param, args);				\
+	_Pragma("GCC diagnostic pop")				\
+								\
+	bpf_snprintf(out, out_size, ___fmt,			\
+		     ___param, sizeof(___param));		\
+})
 
 #endif