Re: [PATCH v2 3/4] Make sure that sorting does not change section order

[Simon Marchi <simon.marchi@polymtl.ca>]

On 2018-06-11 08:08, Petr Tesarik wrote:
> Symbol files may contain multiple sections with the same name.
> Section addresses specified add-symbol-file are assigned to the
> corresponding BFD sections in addr_info_make_relative using sorted
> indexes of both vectors.  Since the sort algorithm is not inherently
> stable, the comparison function uses sectindex to maintain the
> original order.  However, add_symbol_file_command uses zero for all
> sections, so if the user specifies multiple sections with the same
> name, they will be assigned randomly to symbol file sections with
> the same name.
> 
> gdb/ChangeLog:
> 2018-06-11  Petr Tesarik  <ptesarik@suse.com>
> 
> 	* symfile.c (add_symbol_file_command): Make sure that sections
> 	with the same name are sorted in the same order.
> ---
>  gdb/ChangeLog | 5 +++++
>  gdb/symfile.c | 2 +-
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/gdb/ChangeLog b/gdb/ChangeLog
> index 1c5a1f6bfb..0f75992d4c 100644
> --- a/gdb/ChangeLog
> +++ b/gdb/ChangeLog
> @@ -1,5 +1,10 @@
>  2018-06-11  Petr Tesarik  <ptesarik@suse.com>
> 
> +	* symfile.c (add_symbol_file_command): Make sure that sections
> +	with the same name are sorted in the same order.
> +
> +2018-06-11  Petr Tesarik  <ptesarik@suse.com>
> +
>  	* symfile.c (add_symbol_file_command, _initialize_symfile): Do not
>  	require the second argument.  If omitted, load sections at the
>  	addresses specified in the file.
> diff --git a/gdb/symfile.c b/gdb/symfile.c
> index 3e3ab20412..8b8b194334 100644
> --- a/gdb/symfile.c
> +++ b/gdb/symfile.c
> @@ -2185,7 +2185,7 @@ add_symbol_file_command (const char *args, int 
> from_tty)
> 
>        /* Here we store the section offsets in the order they were
>           entered on the command line.  */
> -      section_addrs.emplace_back (addr, sec, 0);
> +      section_addrs.emplace_back (addr, sec, section_addrs.size ());
>        printf_unfiltered ("\t%s_addr = %s\n", sec,
>  			 paddress (gdbarch, addr));

It took me a while to acknowledge that this was correct, because 
other_sections::sectindex usually refers to the section index in the 
BFD.  After digging I understood that this field was actually unused 
until filled by addr_info_make_relative, and that you kind of 
re-purposed it.  It sounds like there should be some comment at 
other_sections::sectindex and probably in add_symbol_file_command to 
explain how it's used.

Another option would be to use std::stable_sort instead of std::sort.  
But it's more resource-hungry and not needed for all paths that lead to 
addrs_section_sort, so it would be a bit wasteful.

Simon