Re: [PATCH] gdb, configure: Add disable-formats option for configure

[Guinevere Larsen <guinevere@redhat.com>]

On 10/7/24 4:17 PM, Eli Zaretskii wrote:
>> Date: Mon, 7 Oct 2024 15:30:37 -0300
>> Cc: gdb-patches@sourceware.org
>> From: Guinevere Larsen <guinevere@redhat.com>
>>
>>> And a more practical question: if I want to build GDB that will run on
>>> GNU/Linux and should be able to debug Linux executables natively and
>>> MS-Windows executables via gdbserver, which formats should I specify
>>> with this new --enable-formats option?
>> Linux uses elf files, and windows uses PE-coff (compiled when coff is
>> requested), so you would need --enable-formats=elf,coff
>>
>> However, with the patch as-is in the list, any value you put in there
>> (including --enable-formats=no, which would add nothing user selected)
>> would compile support for elf and coff anyway, so there is no need to worry.
>>
>> Finally, if you aren't sure, the default behavior if the flag is to
>> compile all readers, keeping with the old behavior.
> I presume users and packagers will want to use this feature to avoid
> compiling in stuff they will never need.  The problem, as I see it, is
> that it is hard to decide what to specify, and your answer to my
> Linux+Windows question illustrates this very well.
>
> By contrast, the security benefits might not be interesting at least
> to some.
That is fine. As the documentation below points out, the default 
behavior is to add all readers just like GDB already does, so if a user 
does not find this feature compelling, they are free to ignore that it 
exists and continue using GDB as they have up until this point.
>
>>> See, I think these are the questions that the readers of the manual
>>> will ask themselves, and we should have the answers there for them.
>>>
>> That's a reasonable point. I don't think we should need to document all
>> targets, but I can see the value of documenting the main usage of the
>> file format when describing the options, as a guideline for users
>> running on mainstream platforms and not meant as an exhaustive list. I'm
>> thinking of adding the following to the readme:
>>
>> `--enable-binary-file-format=FORMAT,FORMAT...'
>> `--enable-binary-file-format=all'
>>     Configure GDB to only be be able to read selected file formats.
>>     The special value "all" causes all file formats to be compiled in,
>> and is the
>>     the default behavior of the option. The other possible options are:
>>     * coff: Main format on windows systems. This is required to compile with
>>       windows target support;
>>     * dbx: (also known as a.out), is a legacy file format, this is
>> recommended
>>       if you know you will be dealing with this file format;
>>     * elf: Main format of linux systems. This is heavily recommended when
>>       compiling with linux support;
>>     * macho: Main format on MacOS systems, this is heavily recommended
>>       when compiling for those targets;
>>     * mips: (also known as ecoff), this is the main file format for targets
>>       running on MIPS CPUs. It is heavily recommended when supporting those
>>       targets.
>>     * xcoff: Main format of AIX systems, this is required to compile for AIX
>>       targets and rs6000 CPUs.
>>
>>
>> What do you think?
> That's a good starting point, but IMO it stops short of answering the
> practical questions people will ask themselves.  For example, how do I
> configure GDB to support a single target that is my native target and
> OS? will specifying the single format from the above list do?

If a user wants to configure their native target, that is related to the 
--target and --enable-target options. Not the file formats. This change 
is not related to how a user decides to support any given CPU or 
operating system, but rather, which binary files they wish GDB to be 
able to understand. If the user knows they want Windows and Linux, but 
don't know how to specify that, they should look at documentation for 
--target and --enable-targets, which is not touched by this patch or 
related to this change whatsoever.

Once they know what target they are running, it will most likely be 
Linux, Windows or MacOS, which are called out by name in this 
description. If it is something different, the user is likely savvy 
enough to look up online what their operating system supports, as there 
are no users of these platforms who do so because it's the default in 
the mainstream machine they bought.

>
> One thing I remember is that many targets produce core files in ELF
> format.  If that is true, then omitting ELF should be discouraged for
> that reason.  And maybe also other similar practical factoids.
>
I believe that is the case because there are many targets that use ELF 
as the objfile format. My internet search couldn't find one very 
reputable source, but it seems that MacOS uses Mach-O format, and 
Windows uses PE/COFF format, the same as the default file format of the 
target.

And once again, users don't need to interact with this feature. If it is 
completely ignored, GDB will continue to work just as it did before this 
patch went in.

-- 
Cheers,
Guinevere Larsen
She/Her/Hers