gdb/build-id: protect against weirdly short build-ids

While looking at build_id_to_bfd_suffix (in gdb/build-id.c) I realised
that GDB would likely not do what we wanted if a build-id was ever a
single byte.

Right now, build-ids generated by the GNU linker are 32 bytes, but
there's nothing that forces this to be the case, it's pretty easy to
create a fake, single byte, build-id.  Given that the build-id is an
external input (read from the objfile), GDB should protect itself
against these edge cases.

The problem with build_id_to_bfd_suffix is that this function creates
the path used to lookup either the debug information, or an
executable, based on its build-id.  So a 3-byte build-id 0xaabbcc will
look in the path: `$DEBUG_FILE_DIRECTORY/.build-id/aa/bbcc.debug`.
However, a single byte build-id 0xaa, will look in the file:
`$DEBUG_FILE_DIRECTORY/.build-id/aa/.debug` which doesn't seem right.

Worse, when looking for an objfile given a build-id GDB will look for
a file called `$DEBUG_FILE_DIRECTORY/.build-id/aa/` with a trailing
'/' character.

I propose that, in build_id_to_bfd_suffix we just return early if the
build-id is 1 byte (or less) with a return value that indicates no
separate file was found.

For testing I made use of the DWARF assembler.  I needed to update the
build-id creation proc, the existing code assumes that the build-id is
a multiple of 4 bytes, so I added some additional padding to ensure
that the generated note was a multiple of 4 bytes, even if the
build-id was not.

I added a test with a 1 byte build-id, and also for the case where the
build-id has zero length.  The zero length case already does what
you'd expect (no debug is loaded) as the bfd library rejects the
build-id when loading it from the objfile, but adding this additional
test is pretty cheap.

Approved-By: Kevin Buettner <kevinb@redhat.com>

1 files changed, 10 insertions, 3 deletions

diff --git a/gdb/testsuite/lib/dwarf.exp b/gdb/testsuite/lib/dwarf.exp
index 1002c75dd09..f6348972968 100644
--- a/gdb/testsuite/lib/dwarf.exp
+++ b/gdb/testsuite/lib/dwarf.exp
@@ -3008,25 +3008,32 @@ namespace eval Dwarf {
 
     proc _note {type name hexdata} {
 	set namelen [expr [string length $name] + 1]
+	set datalen [expr [string length $hexdata] / 2]
 
 	# Name size.
 	_op .4byte $namelen
 	# Data size.
-	_op .4byte [expr [string length $hexdata] / 2]
+	_op .4byte $datalen
 	# Type.
 	_op .4byte $type
 	# The name.
 	_op .ascii [_quote $name]
-	# Alignment.
+	# Alignment (to 4-byte boundary).
 	set align 2
 	set total [expr {($namelen + (1 << $align) - 1) & -(1 << $align)}]
 	for {set i $namelen} {$i < $total} {incr i} {
-	    _op .byte 0
+	    _op .byte 0 padding
 	}
 	# The data.
 	foreach {a b} [split $hexdata {}] {
 	    _op .byte 0x$a$b
 	}
+	# Alignment (to 4-byte boundary).
+	set align 2
+	set total [expr {($datalen + (1 << $align) - 1) & -(1 << $align)}]
+	for {set i $datalen} {$i < $total} {incr i} {
+	    _op .byte 0 padding
+	}
     }
 
     # Emit a note section holding the given build-id.