[PATCH dwarves] dwarves: zero-initialize struct cu in cu__new() to prevent incorrect BTF types

[Alan Maguire <alan.maguire@xxxxxxxxxx>]

BTF deduplication was throwing some strange results, where core kernel
data types were failing to deduplicate due to the return values
of function type members being void (0) instead of the actual type
(unsigned int).  An example of this can be seen below, where
"struct dst_ops" was failing to deduplicate between kernel and
module:

struct dst_ops {
        short unsigned int family;
        unsigned int gc_thresh;
        int (*gc)(struct dst_ops *);
        struct dst_entry * (*check)(struct dst_entry *, __u32);
        unsigned int (*default_advmss)(const struct dst_entry *);
        unsigned int (*mtu)(const struct dst_entry *);
...

struct dst_ops___2 {
        short unsigned int family;
        unsigned int gc_thresh;
        int (*gc)(struct dst_ops___2 *);
        struct dst_entry___2 * (*check)(struct dst_entry___2 *, __u32);
        void (*default_advmss)(const struct dst_entry___2 *);
        void (*mtu)(const struct dst_entry___2 *);
...

This was seen with

bcc648a10cbc ("btf_encoder: Encode DW_TAG_unspecified_type returning routines as void")

...which rewrites the return value as 0 (void) when it is marked
as matching DW_TAG_unspecified_type:

static int32_t btf_encoder__tag_type(struct btf_encoder *encoder, uint32_t type_id_off, uint32_t tag_type)
{
       if (tag_type == 0)
               return 0;

       if (encoder->cu->unspecified_type.tag && tag_type == encoder->cu->unspecified_type.type) {
               // No provision for encoding this, turn it into void.
               return 0;
       }

       return type_id_off + tag_type;
}

However the odd thing was that on further examination, the unspecified type
was not being set, so why was this logic being tripped?  Futher debugging
showed that the encoder->cu->unspecified_type.tag value was garbage, and
the type id happened to collide with "unsigned int"; as a result we
were replacing unsigned ints with void return values, and since this
was being done to function type members in structs, it triggered a
type mismatch which failed deduplication between kernel and module.

The fix is simply to calloc() the cu in cu__new() instead.

Fixes: bcc648a10cbc ("btf_encoder: Encode DW_TAG_unspecified_type returning routines as void")
Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
---
 dwarves.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/dwarves.c b/dwarves.c
index fbebc1d..424381d 100644
--- a/dwarves.c
+++ b/dwarves.c
@@ -626,7 +626,7 @@ struct cu *cu__new(const char *name, uint8_t addr_size,
 		   const unsigned char *build_id, int build_id_len,
 		   const char *filename, bool use_obstack)
 {
-	struct cu *cu = malloc(sizeof(*cu) + build_id_len);
+	struct cu *cu = calloc(1, sizeof(*cu) + build_id_len);
 
 	if (cu != NULL) {
 		uint32_t void_id;
-- 
2.31.1