[PATCH 11/16] dwarf_loader: Handle DW_FORM_block in attr_numeric for Rust discriminant values

[Arnaldo Carvalho de Melo <acme@kernel.org>]

From: Arnaldo Carvalho de Melo <acme@redhat.com>

Rust enums with 128-bit discriminant types (i128/u128) encode their
DW_AT_discr_value as a DW_FORM_block1 containing a 16-byte little-endian
integer, rather than using a standard data form like DW_FORM_data*.

Since attr_numeric did not handle DW_FORM_block*, these discriminant
values were not read and produced 311 warnings on a real Rust binary like
sashiko-cli:

  DW_AT_<0x16>=0xa
  DW_AT_<0x16>=0xa
  ...

(0x16 = DW_AT_discr_value, 0xa = DW_FORM_block1)

Add DW_FORM_block1/block2/block4/block to attr_numeric, reading up to 8
bytes from the block into a uint64_t via memcpy and converting from
little-endian with le64toh() so the result is correct on big-endian hosts
as well.  This covers all practical discriminant values.

Before (sashiko-cli):

  $ pahole --btf_encode sashiko-cli 2>&1 | grep -c 'DW_AT_'
  311

After:

  $ pahole --btf_encode sashiko-cli 2>&1 | wc -l
  0

All warnings from Rust DWARF encoding of sashiko-cli are now resolved.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 dwarf_loader.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/dwarf_loader.c b/dwarf_loader.c
index fc88221ae339e7cd..9f7d2bd23359191b 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -7,6 +7,7 @@
 #include <assert.h>
 #include <dirent.h>
 #include <dwarf.h>
+#include <endian.h>
 #include <elfutils/libdwfl.h>
 #include <elfutils/version.h>
 #include <errno.h>
@@ -353,6 +354,19 @@ static uint64_t attr_numeric(Dwarf_Die *die, uint32_t name)
 			return value;
 	}
 		break;
+	case DW_FORM_block1:
+	case DW_FORM_block2:
+	case DW_FORM_block4:
+	case DW_FORM_block: {
+		Dwarf_Block block;
+		if (dwarf_formblock(&attr, &block) == 0 && block.length > 0) {
+			uint64_t value = 0;
+			size_t n = block.length > sizeof(value) ? sizeof(value) : block.length;
+			memcpy(&value, block.data, n);
+			return le64toh(value);
+		}
+	}
+		break;
 	default:
 		fprintf(stderr, "DW_AT_<0x%x>=0x%x\n", name, form);
 		break;
-- 
2.54.0