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Re: [PATCH RFA] dwarf2read.c: Accommodate older 64-bit DWARF2 format
On Jul 13, 5:43pm, Jim Blandy wrote:
> Approved, if you add discussion like the one included in your post to
> the comment above read_initial_length.
Committed.
Since the patch that I ended up committing is quite different from
what I posted last week, I've appended it below. The bits of actual C
code are identical to the patch I posted last week, but the changes to
the comment are new. In addition to the requested changes, I took the
liberty of updating the comment to point at the current DWARF 3 draft
standard. (The old link was broken due to reality.sgi.com being
decommissioned.)
* dwarf2read.c (read_initial_length): Handle older, non-standard,
64-bit DWARF2 format.
Index: dwarf2read.c
===================================================================
RCS file: /cvs/src/src/gdb/dwarf2read.c,v
retrieving revision 1.62
diff -u -p -r1.62 dwarf2read.c
--- dwarf2read.c 12 Jul 2002 19:55:10 -0000 1.62
+++ dwarf2read.c 16 Jul 2002 20:56:44 -0000
@@ -3863,12 +3863,25 @@ read_address (bfd *abfd, char *buf, cons
return retval;
}
-/* Reads the initial length from a section. The (draft) DWARF 2.1
+/* Read the initial length from a section. The (draft) DWARF 3
specification allows the initial length to take up either 4 bytes
or 12 bytes. If the first 4 bytes are 0xffffffff, then the next 8
bytes describe the length and all offsets will be 8 bytes in length
instead of 4.
+ An older, non-standard 64-bit format is also handled by this
+ function. The older format in question stores the initial length
+ as an 8-byte quantity without an escape value. Lengths greater
+ than 2^32 aren't very common which means that the initial 4 bytes
+ is almost always zero. Since a length value of zero doesn't make
+ sense for the 32-bit format, this initial zero can be considered to
+ be an escape value which indicates the presence of the older 64-bit
+ format. As written, the code can't detect (old format) lengths
+ greater than 4GB. If it becomes necessary to handle lengths somewhat
+ larger than 4GB, we could allow other small values (such as the
+ non-sensical values of 1, 2, and 3) to also be used as escape values
+ indicating the presence of the old format.
+
The value returned via bytes_read should be used to increment
the relevant pointer after calling read_initial_length().
@@ -3879,14 +3892,18 @@ read_address (bfd *abfd, char *buf, cons
[ Note: read_initial_length() and read_offset() are based on the
document entitled "DWARF Debugging Information Format", revision
- 2.1, draft 4, dated July 20, 2000. This document was obtained
+ 3, draft 8, dated November 19, 2001. This document was obtained
from:
- http://reality.sgi.com/dehnert_engr/dwarf/dwarf2p1-draft4-000720.pdf
+ http://reality.sgiweb.org/davea/dwarf3-draft8-011125.pdf
This document is only a draft and is subject to change. (So beware.)
- - Kevin, Aug 4, 2000
+ Details regarding the older, non-standard 64-bit format were
+ determined empirically by examining 64-bit ELF files produced
+ by the SGI toolchain on an IRIX 6.5 machine.
+
+ - Kevin, July 16, 2002
] */
static LONGEST
@@ -3904,6 +3921,18 @@ read_initial_length (bfd *abfd, char *bu
if (cu_header != NULL)
{
cu_header->initial_length_size = 12;
+ cu_header->offset_size = 8;
+ }
+ }
+ else if (retval == 0)
+ {
+ /* Handle (non-standard) 64-bit DWARF2 formats such as that used
+ by IRIX. */
+ retval = bfd_get_64 (abfd, (bfd_byte *) buf);
+ *bytes_read = 8;
+ if (cu_header != NULL)
+ {
+ cu_header->initial_length_size = 8;
cu_header->offset_size = 8;
}
}