[v2,bpf-next,4/7] printk: add type-printing %pT format specifier which uses BTF

diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
index 8ebe46b1..5c66097 100644
--- a/Documentation/core-api/printk-formats.rst
+++ b/Documentation/core-api/printk-formats.rst
@@ -545,6 +545,21 @@  For printing netdev_features_t.
 
 Passed by reference.
 
+BTF-based printing of pointer data
+----------------------------------
+If '%pT' is specified, use the struct btf_ptr * along with kernel vmlinux
+BPF Type Format (BTF) to show the typed data.  For example, specifying
+
+	printk(KERN_INFO "%pT", BTF_PTR_TYPE(skb, struct_sk_buff));
+
+will utilize BTF information to traverse the struct sk_buff * and display it.
+
+Supported modifers are
+ 'c' compact output (no indentation, newlines etc)
+ 'N' do not show type names
+ 'x' show raw pointers (no obfuscation)
+ '0' show zero-valued data (it is not shown by default)
+
 Thanks
 ======
 
diff --git a/include/linux/btf.h b/include/linux/btf.h
index d571125..7b585ab 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -169,10 +169,11 @@  static inline const struct btf_member *btf_type_member(const struct btf_type *t)
 	return (const struct btf_member *)(t + 1);
 }
 
+struct btf *btf_parse_vmlinux(void);
+
 #ifdef CONFIG_BPF_SYSCALL
 const struct btf_type *btf_type_by_id(const struct btf *btf, u32 type_id);
 const char *btf_name_by_offset(const struct btf *btf, u32 offset);
-struct btf *btf_parse_vmlinux(void);
 struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog);
 #else
 static inline const struct btf_type *btf_type_by_id(const struct btf *btf,
diff --git a/include/linux/printk.h b/include/linux/printk.h
index fcde0772..3c3ea53 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -528,4 +528,20 @@  static inline void print_hex_dump_debug(const char *prefix_str, int prefix_type,
 #define print_hex_dump_bytes(prefix_str, prefix_type, buf, len)	\
 	print_hex_dump_debug(prefix_str, prefix_type, 16, 1, buf, len, true)
 
+/**
+ * struct btf_ptr is used for %pT (typed pointer) display; the
+ * additional type string/BTF id are used to render the pointer
+ * data as the appropriate type.
+ */
+struct btf_ptr {
+	void *ptr;
+	const char *type;
+	u32 id;
+};
+
+#define	BTF_PTR_TYPE(ptrval, typeval) \
+	(&((struct btf_ptr){.ptr = ptrval, .type = #typeval}))
+
+#define BTF_PTR_ID(ptrval, idval) \
+	(&((struct btf_ptr){.ptr = ptrval, .id = idval}))
 #endif
diff --git a/lib/Kconfig b/lib/Kconfig
index 5d53f96..ac3a513 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -6,6 +6,22 @@ 
 config BINARY_PRINTF
 	def_bool n
 
+config BTF_PRINTF
+	bool "print type information using BPF type format"
+	depends on DEBUG_INFO_BTF
+	default n
+	help
+	  Print structures, unions etc pointed to by pointer argument using
+	  printk() family of functions (vsnprintf, printk, trace_printk, etc).
+	  For example, we can specify
+	  printk(KERN_INFO, "%pT<struct sk_buff>", skb); to print the skb
+	  data structure content, including all nested type data.
+	  Pointers within data structures displayed are not followed, and
+	  are obfuscated where specified in line with normal pointer display.
+	  via printk.
+
+	  Depends on availability of vmlinux BTF information.
+
 menu "Library routines"
 
 config RAID6_PQ
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 7c488a1..f9276f8 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -43,6 +43,7 @@ 
 #ifdef CONFIG_BLOCK
 #include <linux/blkdev.h>
 #endif
+#include <linux/btf.h>
 
 #include "../mm/internal.h"	/* For the trace_print_flags arrays */
 
@@ -2059,6 +2060,103 @@  char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
 	return widen_string(buf, buf - buf_start, end, spec);
 }
 
+#if IS_ENABLED(CONFIG_BTF_PRINTF)
+#define btf_modifier_flag(c)	(c == 'c' ? BTF_SHOW_COMPACT :	\
+				 c == 'N' ? BTF_SHOW_NONAME :	\
+				 c == 'x' ? BTF_SHOW_PTR_RAW :	\
+				 c == '0' ? BTF_SHOW_ZERO : 0)
+
+static noinline_for_stack
+char *btf_string(char *buf, char *end, void *ptr, struct printf_spec spec,
+		 const char *fmt)
+{
+	struct btf_ptr *bp = (struct btf_ptr *)ptr;
+	u8 btf_kind = BTF_KIND_TYPEDEF;
+	const struct btf_type *t;
+	const struct btf *btf;
+	char *buf_start = buf;
+	const char *btf_type;
+	u64 flags = 0, mod;
+	s32 btf_id;
+
+	if (check_pointer(&buf, end, ptr, spec))
+		return buf;
+
+	if (check_pointer(&buf, end, bp->ptr, spec))
+		return buf;
+
+	while (isalnum(*fmt)) {
+		mod = btf_modifier_flag(*fmt);
+		if (!mod)
+			break;
+		flags |= mod;
+		fmt++;
+	}
+
+	btf = bpf_get_btf_vmlinux();
+	if (IS_ERR_OR_NULL(btf))
+		return ptr_to_id(buf, end, bp->ptr, spec);
+
+	if (bp->type != NULL) {
+		btf_type = bp->type;
+
+		if (strncmp(bp->type, "struct ", strlen("struct ")) == 0) {
+			btf_kind = BTF_KIND_STRUCT;
+			btf_type += strlen("struct ");
+		} else if (strncmp(btf_type, "union ", strlen("union ")) == 0) {
+			btf_kind = BTF_KIND_UNION;
+			btf_type += strlen("union ");
+		} else if (strncmp(btf_type, "enum ", strlen("enum ")) == 0) {
+			btf_kind = BTF_KIND_ENUM;
+			btf_type += strlen("enum ");
+		}
+
+		if (strlen(btf_type) == 0)
+			return ptr_to_id(buf, end, bp->ptr, spec);
+
+		/*
+		 * Assume type specified is a typedef as there's not much
+		 * benefit in specifying int types other than wasting time
+		 * on BTF lookups; we optimize for the most useful path.
+		 *
+		 * Fall back to BTF_KIND_INT if this fails.
+		 */
+		btf_id = btf_find_by_name_kind(btf, btf_type, btf_kind);
+		if (btf_id < 0)
+			btf_id = btf_find_by_name_kind(btf, btf_type,
+						       BTF_KIND_INT);
+	} else if (bp->id > 0)
+		btf_id = bp->id;
+	else
+		return ptr_to_id(buf, end, bp->ptr, spec);
+
+	if (btf_id > 0)
+		t = btf_type_by_id(btf, btf_id);
+	if (btf_id <= 0 || !t)
+		return ptr_to_id(buf, end, bp->ptr, spec);
+
+	buf += btf_type_snprintf_show(btf, btf_id, bp->ptr, buf,
+				      end - buf_start, flags);
+
+	return widen_string(buf, buf - buf_start, end, spec);
+}
+#else
+static noinline_for_stack
+char *btf_string(char *buf, char *end, void *ptr, struct printf_spec spec,
+	const char *fmt)
+{
+	struct btf_ptr *bp = (struct btf_ptr *)ptr;
+
+	if (check_pointer(&buf, end, ptr, spec))
+		return buf;
+
+	if (check_pointer(&buf, end, bp->ptr, spec))
+		return buf;
+
+	return ptr_to_id(buf, end, bp->ptr, spec);
+}
+#endif /* IS_ENABLED(CONFIG_BTF_PRINTF) */
+
 /*
  * Show a '%p' thing.  A kernel extension is that the '%p' is followed
  * by an extra set of alphanumeric characters that are extended format
@@ -2169,6 +2267,19 @@  char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
  *		P node name, including a possible unit address
  * - 'x' For printing the address. Equivalent to "%lx".
  *
+ * - 'T[cNx0]' For printing struct btf_ptr * data using BPF Type Format (BTF).
+ *
+ *			Optional arguments are
+ *			c		compact (no indentation/newlines)
+ *			N		do not print type and member names
+ *			x		do not obfuscate pointers
+ *			0		show 0-valued data
+ *
+ *    BPF_PTR_TYPE(ptr, type) can be used to place pointer and type string
+ *    in the "struct btf_ptr *" expected; for example:
+ *
+ *	printk(KERN_INFO "%pT", BTF_PTR_TYPE(skb, struct sk_buff));
+ *
  * ** When making changes please also update:
  *	Documentation/core-api/printk-formats.rst
  *
@@ -2251,6 +2362,8 @@  char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 		if (!IS_ERR(ptr))
 			break;
 		return err_ptr(buf, end, ptr, spec);
+	case 'T':
+		return btf_string(buf, end, ptr, spec, fmt + 1);
 	}
 
 	/* default is to _not_ leak addresses, hash before printing */