Fill between executable sections with trap instructions

[rui314]

Bugzilla Link	32227
Resolution	FIXED
Resolved on	Apr 07, 2017 05:27
Version	unspecified
OS	All
CC	@emaste,@jh7370

Extended Description

Currently, LLD leaves padding between executable sections as-is, so they are 0. That is not desirable for a few reasons.

1. Disassemblers would interpret the sequences of 0 bytes at end of each function as machine instructions and print out bogus disassembly.

2. Leaving executable 0 bytes could potentially make some attack easier.

We want to fill gaps with trap instructions. On x86 and x86-64, that is 0xcc, for example.

[rui314]

assigned to @jh7370

[llvmbot]

Interesting that gold/bfd implementation is slightly more complicated than just adding some gap bytes.

For example if you have just a nop in a section that is 32 bytes size:
_start
nop

output can be:
080481a0 <_start>:
80481a0: 90 nop
80481a1: e9 1a 00 00 00 jmp 80481c0 <_start+0x20>
80481a6: 90 nop

So they even modify code to insert jump here and skip the nops. I do not know the reason why they do all that, but that was done from initial commit for x64: "From Craig Silverstein: x86_64 support." (https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=gold/x86_64.cc;hb=2e30d253b71680ea2f09203ad412f0eb7d3f7ff3#l1531).

I think filling with some trap bytes should be enough for start. Since we anyways did not have valid nops here.

[llvmbot]

Patch: D30901

[jh7370]

George and I started working on this independently before I had bugzilla access and my patch ended up for review first, so we are working on that one now instead - D30886.

[jh7370]

Fixed in r299755/D30886.