Clang 12 in opt mode produces a couple of assembly patterns that were
not handled by delocate.
Firstly, two-digit vector indexes were just a simple omission. Fixed.
Secondly, Clang puts symbol deltas in .byte directives, and bit-shifts
them. The .byte directive was not considered to be a symbol-containing
directive because it's too small, but it could store deltas.
Additionally, bit-shifting of symbol expressions was not supported.
Fixed.
Change-Id: I796299821f5ac7d3639fa6243c5d9bd5342bbddf
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/47064
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Add Aarch64 support to delocate. Since it's a modern ISA, it's actually
not too bad once I understood the behaviour of the assembler.
Change-Id: I105fede43b5196b7ff7bdbf1ee71c6cfa2fc1aab
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/44848
Reviewed-by: David Benjamin <davidben@google.com>
Aarch64 assembly is quite different from x86-64 or POWER. But the system
of directives is the same so there's quite a lot of utility from being
able to use the same delocate framework.
Unfortunately, with peg, there's no obvious way to be able to parse
instructions differently without breaking the parsing into two stages.
Thus the parser is extended here to support all three ISAs. This seems
to work ok without breaking either of the other two.
Change-Id: Iced0f651e556e6ffae3eb35f2edfc0bf84167967
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/44846
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Large memory models on x86-64 allow the code/data of a shared object /
executable to be larger than 2GiB. This is typically impossible because
x86-64 code frequently uses int32 offsets from RIP.
Consider the following program:
int getpid();
int main() {
return getpid();
}
This is turned into the following assembly under a large memory model:
.L0$pb:
leaq .L0$pb(%rip), %rax
movabsq $_GLOBAL_OFFSET_TABLE_-.L0$pb, %rcx
addq %rax, %rcx
movabsq $getpid@GOT, %rdx
xorl %eax, %eax
jmpq *(%rcx,%rdx) # TAILCALL
And, with relocations:
0: 48 8d 05 f9 ff ff ff lea -0x7(%rip),%rax # 0 <main>
7: 48 b9 00 00 00 00 00 movabs $0x0,%rcx
e: 00 00 00
9: R_X86_64_GOTPC64 _GLOBAL_OFFSET_TABLE_+0x9
11: 48 01 c1 add %rax,%rcx
14: 48 ba 00 00 00 00 00 movabs $0x0,%rdx
1b: 00 00 00
16: R_X86_64_GOT64 getpid
1e: 31 c0 xor %eax,%eax
20: ff 24 11 jmpq *(%rcx,%rdx,1)
We can see that, in the large memory model, function calls involve
loading the address of _GLOBAL_OFFSET_TABLE_ (using `movabs`, which
takes a 64-bit immediate) and then indexing into it. Both cause
relocations.
If we link the binary and disassemble we get:
0000000000001120 <main>:
1120: 48 8d 05 f9 ff ff ff lea -0x7(%rip),%rax # 1120 <main>
1127: 48 b9 e0 2e 00 00 00 movabs $0x2ee0,%rcx
112e: 00 00 00
1131: 48 01 c1 add %rax,%rcx
1134: 48 ba d8 ff ff ff ff movabs $0xffffffffffffffd8,%rdx
113b: ff ff ff
113e: 31 c0 xor %eax,%eax
1140: ff 24 11 jmpq *(%rcx,%rdx,1)
Thus the _GLOBAL_OFFSET_TABLE_ symbol is at 0x1120+0x2ee0 = 0x4000.
That's the address of the .got.plt section. But the offset “into” the
table is -0x40, putting it at 0x3fd8, in .got:
Idx Name Size VMA LMA File off Algn
18 .got 00000030 0000000000003fd0 0000000000003fd0 00002fd0 2**3
19 .got.plt 00000018 0000000000004000 0000000000004000 00003000 2**3
And, indeed, there's a dynamic relocation to setup that address:
OFFSET TYPE VALUE
0000000000003fd8 R_X86_64_GLOB_DAT getpid@GLIBC_2.2.5
Accessing data or BSS works the same: the address of the variable is
stored relative to _GLOBAL_OFFSET_TABLE_.
This is a bit of a pain because we want to delocate the module into a
single .text segment so that it moves through linking unaltered. If we
took the obvious path and built our own offset table then it would need
to contain absolute addresses, but they are only available at runtime
and .text segments aren't supposed to be run-time patched. (That's why
.rela.dyn is a separate segment.) If we use a different segment then
we have the same problem as with the original offset table: the offset
to the segment is unknown when compiling the module.
Trying to pattern match this two-step lookup to do extensive rewriting
seems fragile: I'm sure the compilers will move things around and
interleave other work in time, if they don't already.
So, in order to handle movabs trying to load _GLOBAL_OFFSET_TABLE_ we
define a symbol in the same segment, but outside of the hashed region of
the module, that contains the offset from that position to
_GLOBAL_OFFSET_TABLE_:
.boringssl_got_delta:
.quad _GLOBAL_OFFSET_TABLE_-.boringssl_got_delta
Then a movabs of $_GLOBAL_OFFSET_TABLE_-.Lfoo turns into:
movq .boringssl_got_delta(%rip), %destreg
addq $.boringssl_got_delta-.Lfoo, %destreg
This works because it's calculating
_GLOBAL_OFFSET_TABLE_ - got_delta + (got_delta - .Lfoo)
When that value is added to .Lfoo, as the original code will do, the
correct address results. Also it doesn't need an extra register because
we know that 32-bit offsets are sufficient for offsets within the
module.
As for the offsets within the offset table, we have to load them from
locations outside of the hashed part of the module to get the
relocations out of the way. Again, no extra registers are needed.
Change-Id: I87b19a2f8886bd9f7ac538fd55754e526bcf3097
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/42324
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Adam Langley