https://sourceware.org/bugzilla/show_bug.cgi?id=24546
Bug ID: 24546
Summary: x86-64 far jump/call encoding issues
Product: binutils
Version: 2.32
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: gas
Assignee: unassigned at sourceware dot org
Reporter: andrew.cooper3 at citrix dot com
Target Milestone: ---
I have some problems when trying to encode the 64bit forms of far call/jump.
Like all other far operations in 64bit, lcall/ljmp default to a 32bit operand,
and require a rex64 to promote the instruction to having a 64bit operand
(specifically, it changes these instructions between having a 6 byte operand
and a 10 byte operand).
$ cat far-jmps.S
.code64
code64:
lcalll *(%rsp)
rex64 lcall *(%rsp)
ljmpl *(%rsp)
rex64 ljmp *(%rsp)
lretl
lretq
This version of the file assembles correct:
$ /local/bin/bin-2.32/bin/as far-jmps.S -o far-jmps.o
$
However, when substituting the rex64 prefix for an l or q suffix, assembly
fails with:
$ /local/bin/bin-2.32/bin/as far-jmps.S -o far-jmps.o
far-jmps.S: Assembler messages:
far-jmps.S:4: Error: invalid instruction suffix for `lcall'
far-jmps.S:7: Error: invalid instruction suffix for `ljmp'
Furthermore (or possibly relatedly), objdump doesn't disassemble the
instruction in an expected manner:
$ /local/bin/bin-2.32/bin/objdump -d far-jmps.o
far-jmps.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <code64>:
0: ff 1c 24 lcall *(%rsp)
3: 48 ff 1c 24 rex.W lcall *(%rsp)
7: ff 2c 24 ljmp *(%rsp)
a: 48 ff 2c 24 rex.W ljmp *(%rsp)
e: cb lret
f: 48 cb lretq
The rex.W prefix printed is an accurate representation of the encoding, but an
l or q suffix would be the consistent way of rendering the instructions.
Observe that throughout all of this, lret and lretq do assemble and disassemble
in the expected manner (as do iretl and iretq which I omitted from the
example.)
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