Re: [PATCH 1/2] gdb: fix use of uninitialised gdb_printing_disassembler::m_in_comment

[Simon Marchi <simark@simark.ca>]

On 2022-07-20 09:14, Andrew Burgess via Gdb-patches wrote:
> Simon pointed out that gdb_printing_disassembler::m_in_comment can be
> used uninitialised by the Python disassembler API code.  This issue
> was spotted when GDB was built with the undefined behaviour sanitizer,
> and causes the gdb.python/py-disasm.exp test to fail like this:
> 
>   (gdb) PASS: gdb.python/py-disasm.exp: global_disassembler=GlobalPreInfoDisassembler: python add_global_disassembler(GlobalPreInfoDisassembler)
>   disassemble main
>   Dump of assembler code for function main:
>      0x0000555555555119 <+0>:     push   %rbp
>      0x000055555555511a <+1>:     mov    %rsp,%rbp
>      0x000055555555511d <+4>:     nop
>   /home/user/src/binutils-gdb/gdb/disasm.h:144:12: runtime error: load of value 118, which is not a valid value for type 'bool'
> 
> The problem is that in disasmpy_builtin_disassemble we create a new
> instance of gdbpy_disassembler, which is a sub-class of
> gdb_printing_disassembler, however, the m_in_comment field is never
> initialised.
> 
> This commit fixes the issue by providing a default initialisation
> value for m_in_comment in disasm.h.  As we only ever disassemble a
> single instruction in disasmpy_builtin_disassemble then we don't need
> to worry about reseting m_in_comment back to false after the single
> instruction has been disassembled.
> 
> With this commit the above issue is resolved and
> gdb.python/py-disasm.exp now passes.
> ---
>  gdb/disasm.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/gdb/disasm.h b/gdb/disasm.h
> index 2921d537e0a..09cb3921767 100644
> --- a/gdb/disasm.h
> +++ b/gdb/disasm.h
> @@ -166,7 +166,7 @@ struct gdb_printing_disassembler : public gdb_disassemble_info
>       uses styled output and emits a start of comment character.  It is up
>       to the code that uses this disassembler class to reset this flag back
>       to false at a suitable time (e.g. at the end of every line).  */
> -  bool m_in_comment;
> +  bool m_in_comment = false;
>  };
>  
>  /* A basic disassembler that doesn't actually print anything.  */

Thanks, LGTM.

Simon