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

[Joe Perches <joe@perches.com>]

On Wed, 2020-05-13 at 16:22 -0700, Joe Perches wrote:
> On Wed, 2020-05-13 at 16:07 -0700, Alexei Starovoitov wrote:
> > On Wed, May 13, 2020 at 4:05 PM Joe Perches <joe@perches.com> wrote:
> > > On Tue, 2020-05-12 at 06:56 +0100, Alan Maguire wrote:
> > > > printk supports multiple pointer object type specifiers (printing
> > > > netdev features etc).  Extend this support using BTF to cover
> > > > arbitrary types.  "%pT" specifies the typed format, and the pointer
> > > > argument is a "struct btf_ptr *" where struct btf_ptr is as follows:
> > > > 
> > > > struct btf_ptr {
> > > >       void *ptr;
> > > >       const char *type;
> > > >       u32 id;
> > > > };
> > > > 
> > > > Either the "type" string ("struct sk_buff") or the BTF "id" can be
> > > > used to identify the type to use in displaying the associated "ptr"
> > > > value.  A convenience function to create and point at the struct
> > > > is provided:
> > > > 
> > > >       printk(KERN_INFO "%pT", BTF_PTR_TYPE(skb, struct sk_buff));
> > > > 
> > > > When invoked, BTF information is used to traverse the sk_buff *
> > > > and display it.  Support is present for structs, unions, enums,
> > > > typedefs and core types (though in the latter case there's not
> > > > much value in using this feature of course).
> > > > 
> > > > Default output is indented, but compact output can be specified
> > > > via the 'c' option.  Type names/member values can be suppressed
> > > > using the 'N' option.  Zero values are not displayed by default
> > > > but can be using the '0' option.  Pointer values are obfuscated
> > > > unless the 'x' option is specified.  As an example:
> > > > 
> > > >   struct sk_buff *skb = alloc_skb(64, GFP_KERNEL);
> > > >   pr_info("%pT", BTF_PTR_TYPE(skb, struct sk_buff));
> > > > 
> > > > ...gives us:
> > > > 
> > > > (struct sk_buff){
> > > >  .transport_header = (__u16)65535,
> > > >        .mac_header = (__u16)65535,
> > > >  .end = (sk_buff_data_t)192,
> > > >  .head = (unsigned char *)000000006b71155a,
> > > >  .data = (unsigned char *)000000006b71155a,
> > > >  .truesize = (unsigned int)768,
> > > >  .users = (refcount_t){
> > > >   .refs = (atomic_t){
> > > >    .counter = (int)1,
> > > 
> > > Given
> > > 
> > >   #define BTF_INT_ENCODING(VAL)   (((VAL) & 0x0f000000) >> 24)
> > > 
> > > Maybe
> > > 
> > >   #define BTF_INT_SIGNED  (1 << 0)
> > >   #define BTF_INT_CHAR    (1 << 1)
> > >   #define BTF_INT_BOOL    (1 << 2)
> > > 
> > > could be extended to include
> > > 
> > >   #define BTF_INT_HEX     (1 << 3)
> > > 
> > > So hex values can be appropriately pretty-printed.
> > 
> > Nack to that.
> 
> why?
> 

Tell me what's wrong with the idea.

Here's a possible implementation:
---
 Documentation/bpf/btf.rst      |  5 +++--
 include/uapi/linux/btf.h       |  1 +
 kernel/bpf/btf.c               |  5 ++++-
 tools/bpf/bpftool/btf.c        |  2 ++
 tools/bpf/bpftool/btf_dumper.c | 13 +++++++++++++
 tools/include/uapi/linux/btf.h |  1 +
 6 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
index 4d565d202ce3..56aaa189e7fb 100644
--- a/Documentation/bpf/btf.rst
+++ b/Documentation/bpf/btf.rst
@@ -139,10 +139,11 @@ The ``BTF_INT_ENCODING`` has the following attributes::
   #define BTF_INT_SIGNED  (1 << 0)
   #define BTF_INT_CHAR    (1 << 1)
   #define BTF_INT_BOOL    (1 << 2)
+  #define BTF_INT_HEX     (1 << 3)
 
 The ``BTF_INT_ENCODING()`` provides extra information: signedness, char, or
-bool, for the int type. The char and bool encoding are mostly useful for
-pretty print. At most one encoding can be specified for the int type.
+bool, for the int type. The char, bool and hex encodings are mostly useful
+for pretty print. At most one encoding can be specified for the int type.
 
 The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int
 type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4.
diff --git a/include/uapi/linux/btf.h b/include/uapi/linux/btf.h
index 5a667107ad2c..36f309209786 100644
--- a/include/uapi/linux/btf.h
+++ b/include/uapi/linux/btf.h
@@ -90,6 +90,7 @@ struct btf_type {
 #define BTF_INT_SIGNED	(1 << 0)
 #define BTF_INT_CHAR	(1 << 1)
 #define BTF_INT_BOOL	(1 << 2)
+#define BTF_INT_HEX	(1 << 3)
 
 /* BTF_KIND_ENUM is followed by multiple "struct btf_enum".
  * The exact number of btf_enum is stored in the vlen (of the
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 58c9af1d4808..90bdc0635321 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -501,6 +501,8 @@ static const char *btf_int_encoding_str(u8 encoding)
 		return "CHAR";
 	else if (encoding == BTF_INT_BOOL)
 		return "BOOL";
+	else if (encoding == BTF_INT_HEX)
+		return "HEX";
 	else
 		return "UNKN";
 }
@@ -1404,7 +1406,8 @@ static s32 btf_int_check_meta(struct btf_verifier_env *env,
 	if (encoding &&
 	    encoding != BTF_INT_SIGNED &&
 	    encoding != BTF_INT_CHAR &&
-	    encoding != BTF_INT_BOOL) {
+	    encoding != BTF_INT_BOOL &&
+	    encoding != BTF_INT_HEX) {
 		btf_verifier_log_type(env, t, "Unsupported encoding");
 		return -ENOTSUPP;
 	}
diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index 41a1346934a1..44a129c40873 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -59,6 +59,8 @@ static const char *btf_int_enc_str(__u8 encoding)
 		return "CHAR";
 	case BTF_INT_BOOL:
 		return "BOOL";
+	case BTF_INT_HEX:
+		return "HEX";
 	default:
 		return "UNKN";
 	}
diff --git a/tools/bpf/bpftool/btf_dumper.c b/tools/bpf/bpftool/btf_dumper.c
index ede162f83eea..96947ef92565 100644
--- a/tools/bpf/bpftool/btf_dumper.c
+++ b/tools/bpf/bpftool/btf_dumper.c
@@ -418,6 +418,19 @@ static int btf_dumper_int(const struct btf_type *t, __u8 bit_offset,
 	case BTF_INT_BOOL:
 		jsonw_bool(jw, *(int *)data);
 		break;
+	case BTF_INT_HEX:
+		if (BTF_INT_BITS(*int_type) == 64)
+			jsonw_printf(jw, "%llx", *(long long *)data);
+		else if (BTF_INT_BITS(*int_type) == 32)
+			jsonw_printf(jw, "%x", *(int *)data);
+		else if (BTF_INT_BITS(*int_type) == 16)
+			jsonw_printf(jw, "%hx", *(short *)data);
+		else if (BTF_INT_BITS(*int_type) == 8)
+			jsonw_printf(jw, "%hhx", *(char *)data);
+		else
+			btf_dumper_int_bits(*int_type, bit_offset, data, jw,
+					    is_plain_text);
+		break;
 	default:
 		/* shouldn't happen */
 		return -EINVAL;
diff --git a/tools/include/uapi/linux/btf.h b/tools/include/uapi/linux/btf.h
index 5a667107ad2c..36f309209786 100644
--- a/tools/include/uapi/linux/btf.h
+++ b/tools/include/uapi/linux/btf.h
@@ -90,6 +90,7 @@ struct btf_type {
 #define BTF_INT_SIGNED	(1 << 0)
 #define BTF_INT_CHAR	(1 << 1)
 #define BTF_INT_BOOL	(1 << 2)
+#define BTF_INT_HEX	(1 << 3)
 
 /* BTF_KIND_ENUM is followed by multiple "struct btf_enum".
  * The exact number of btf_enum is stored in the vlen (of the