The following GDB behaviour was observed:
(gdb) x/1i 0x0001014a
0x1014a <main+8>: jal 0x10132 <foo>
(gdb) show architecture
The target architecture is set automatically (currently riscv:rv32)
(gdb) set architecture riscv:rv32
The target architecture is assumed to be riscv:rv32
(gdb) x/1i 0x0001014a
0x1014a <main+8>: 0x37e5
(gdb)
Notice that initially we can disassemble the instruction (it's a
compressed jal instruction), but after setting the architecture we can
no longer disassemble the instruction.
This is particularly puzzling as GDB initially thought the
architecture was 'riscv:rv32', but when we force the architecture to
be that, the disassembly stops working.
This issue was introduced with this commit:
commit c35d018b1a5ec604e49a807402c4205530b25ca8
Date: Mon Jan 27 15:19:30 2020 -0800
RISC-V: Fix gdbserver problem with handling arch strings.
In this commit we try to make riscv_scan handle cases where we see
architecture strings like 'riscv:rv32imc' (for example). Normally
this wouldn't match as bfd_default_scan requires an exact match, so we
extended riscv_scan to ignore trailing characters.
Unfortunately the default riscv arch is called 'riscv', is 64-bit,
and has its mach type set to 0, which I think is intended to pair with
code is riscv-dis.c:riscv_disassemble_insn that tries to guess if we
are 32 or 64 bit.
What happens then is that 'riscv:rv32' is first tested against 'riscv'
using bfd_default_scan, this doesn't match, we then compare this to
'riscv', but allowing trailing characters to be ignored, this matches,
and our 'riscv:rv32' matches against the default (64-bit)
architecture.
The solution I propose is to prevent the default architecture from
taking part in this "ignore trailing characters" extra match case,
only the more specific 'riscv:rv32' and 'riscv:rv64' get this extra
matching.
bfd/ChangeLog:
* cpu-riscv.c (riscv_scan): Don't allow shorter matches using the
default architecture.
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