Pretty-printers auto-loading and static libraries

Pretty-printers auto-loading and static libraries

[Pierre-Marie de Rodat]

Hi,

I'm trying to write GDB/Python pretty-printers for various data 
structures (like standard containers) from the GNAT runtime. My 
experiments are going quite well, it's quite hopeful! However GDB 
auto-loading features don't seem to be appropriate for this since most 
of the time the GNAT runtime is statically linked into programs.

Automatic scripts loading for statically linked libraries in general is 
a feature that looks quite hard to support[1]. However the most common 
use case for Ada should be fairly easy to catch: if the program contains 
the "adainit" symbol, then it is highly likely that it's a program whose 
entry point is in Ada, and thus that the GNAT runtime pretty-printers 
should be enabled.

I guess my pretty-printers should be temptatively loaded when the 
new_objfile event is triggered: if the corresponding Objfile defines an 
"adainit" symbol they should be registered in it. Unfortunately I failed 
so far to implement this behavior: Objfile do not expose the 
corresponding symbol table and Symtab_and_line instances don't even 
enable one to perform symbol lookups.

I'm not familiar with GDB internals yet, so what do you think about 
this? Would it be reasonable to extend the Python API to enable one to 
do so? Or does someone have another idea to load these pretty-printers 
only when needed?

Thank you in advance!

[1] And it looks like this is not a recent discovery: 
<https://sourceware.org/ml/archer/2008-q4/msg00168.html>

-- 
Pierre-Marie de Rodat

Re: Pretty-printers auto-loading and static libraries

[Pierre-Marie de Rodat]

John,

On 01/14/2015 03:21 AM, John Gilmore wrote:
> People who want to debug Ada programs with this pretty-printer can
> create a .gdbinit in their working directory, or in their home
> directory, that loads any pretty-printer that they want.

Yes, I am aware that automatic scripts loading features have security 
implications and I don't intend to introduce security breaches into GDB. 
The point is that GDB already has such auto loading facilities and ways 
to restrict them[1]. As far as I could see, these are even enabled by 
default on most distributions.

What I would like to do is to leverage this so that pretty-printers for 
Ada work out of the box, otherwise nobody will remember to use them (or 
will bother configuring things).

Besides, if I understand correctly, auto-loading pretty-printers, type 
printers and so on *only when useful* is important for performance 
reasons. If one has to configure GDB to load scripts for all the 
runtime/libraries he/she uses, this is going to make GDB's 
initialization longer and will likely make regular work longer because 
of lookups.

I also guess doing this would pollute the output for "info 
pretty-print", making it harder to enable/disable specific 
pretty-printers. I don't want to have Ada pretty-printers listed when 
I'm debugging some C++ only application. ;-)

[1] 
https://sourceware.org/gdb/onlinedocs/gdb/Auto_002dloading-safe-path.html#Auto_002dloading-safe-path

-- 
Pierre-Marie de Rodat

Re: Pretty-printers auto-loading and static libraries

[Pierre-Marie de Rodat]

On 01/13/2015 04:05 PM, Pierre-Marie de Rodat wrote:
> I guess my pretty-printers should be temptatively loaded when the
> new_objfile event is triggered: if the corresponding Objfile defines an
> "adainit" symbol they should be registered in it. Unfortunately I failed
> so far to implement this behavior: Objfile do not expose the
> corresponding symbol table and Symtab_and_line instances don't even
> enable one to perform symbol lookups.

I dived into the Python API documentation with a fresh mind:

   - gdb.lookup_global_symbol returns Symbol instances;
   - Symbol instances have a symtab attribute;
   - Symtab instances have an objfile attribute.

So it is actually possible to register pretty-printers depending on the 
presence of a symbol! I tried it with my setups and it works properly. :-)

Still to be determined: I don't know if performing a global symbol 
lookup at each objfile load is acceptable from a performance point of view.

-- 
Pierre-Marie de Rodat

Re: Pretty-printers auto-loading and static libraries

[Pierre-Marie de Rodat]

Doug,

On 01/31/2015 09:15 PM, Doug Evans wrote:
> One thing gdb uses to do auto-loading of files is to look for a special
> section named .debug_gdb_scripts in the executable/shared-library
> and if present then it loads the scripts mentioned there.
> The current release supports listing files to load in .debug_gdb_scripts.
> Top-of-tree now supports putting the script into .debug_gdb_scripts.

That's good to know, thanks!

-- 
Pierre-Marie de Rodat