[PATCH v3 1/1] gdb: abort start command if there is no symbol for 'main_name ()'

[Luis Machado]

On 12/13/24 07:39, Rohr, Stephan wrote:
> Hi Luis,
> 
> thanks for the hint, the patch indeed causes a regression if symbols are stripped.

No worries.

> 
> What about changing to:
> 
>   struct symbol *sym
>     = lookup_symbol_search_name (main_name (), nullptr,
> 				 SEARCH_FUNCTION_DOMAIN).symbol;
> 
>   if (sym == nullptr && !have_minimal_symbols (current_program_space))
>     error (_("No symbol table loaded.  Use the \"file\" command."));
> 
> This fixes the Windows issue I originally observed.  Also, Ada doesn't regress
> anymore.

I think it might work, though it skips the minimal symbols check whenever we return
a non-null sym. I'll defer to Tromey to confirm this is enough for Ada to work
correctly.

> 
> Thanks
> Stephan
> 
>> -----Original Message-----
>> From: Luis Machado <luis.machado@arm.com>
>> Sent: Wednesday, 11 December 2024 10:25
>> To: Rohr, Stephan <stephan.rohr@intel.com>; gdb-patches@sourceware.org
>> Cc: guinevere@redhat.com; eliz@gnu.org; Tom Tromey <tom@tromey.com>
>> Subject: Re: [PATCH v3 1/1] gdb: abort start command if there is no symbol for
>> 'main_name ()'
>>
>> On 12/11/24 08:41, Stephan Rohr wrote:
>>> From: "Rohr, Stephan" <stephan.rohr@intel.com>
>>>
>>> GDB aborts the 'start' command if the minimal symbols cannot be
>>> resolved.  On Windows, GDB reads the minimal symbols from the COFF
>>> header of the PE file.  The symbol table is deprecated and the
>>> number of symbols in the COFF header may be zero [1].  For example,
>>> the LLVM clang compiler for Windows MSVC can be instructed to generate
>>> DWARF:
>>>
>>>   clang++ -g -O0 -gdwarf -fuse-ld=lld test.cpp -o test_clang
>>>
>>> The corresponding COFF file header shows:
>>>
>>>   FILE HEADER VALUES
>>>         8664 machine (x64)
>>>            E number of sections
>>>     66E889EC time date stamp Mon Sep 16 21:41:32 2024
>>>        FB400 file pointer to symbol table
>>>            0 number of symbols
>>>           F0 size of optional header
>>>           22 characteristics
>>>
>>> GDB is not able to read the minimal symbols; the `start' command fails
>>> with an error:
>>>
>>>   (gdb) start
>>>   No symbol table loaded.  Use the "file" command.
>>>
>>> Manually inserting a breakpoint in main works fine:
>>>
>>>   (gdb) tbreak main
>>>   Temporary breakpoint 1 at 0x14000100c: file test.cpp, line 6.
>>>   (gdb) run
>>>   Starting program: C:\test-clang
>>>
>>>   Temporary breakpoint 1, main () at test.cpp:6
>>>   6         std::cout << "Hello World.\n";
>>>
>>> Fix this by testing if a symbol for `main' is available instead of checking
>>> the minimal symbol table:
>>>
>>>   (gdb) start
>>>   Temporary breakpoint 1 at 0x14000100c: file test.cpp, line 6.
>>>   Starting program: C:\test-clang
>>>
>>>   Temporary breakpoint 1, main () at test.cpp:6
>>>   6         std::cout << "Hello World.\n";
>>>   (gdb)
>>>
>>> [1]: https://learn.microsoft.com/en-us/windows/win32/debug/pe-format
>>>
>>> Reviewed-By: Guinevere Larsen <guinevere@redhat.com>
>>> ---
>>>  gdb/infcmd.c | 10 ++++++----
>>>  1 file changed, 6 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/gdb/infcmd.c b/gdb/infcmd.c
>>> index 5c0e3f51162..3c9d6aff9a4 100644
>>> --- a/gdb/infcmd.c
>>> +++ b/gdb/infcmd.c
>>> @@ -516,10 +516,12 @@ run_command (const char *args, int from_tty)
>>>  static void
>>>  start_command (const char *args, int from_tty)
>>>  {
>>> -  /* Some languages such as Ada need to search inside the program
>>> -     minimal symbols for the location where to put the temporary
>>> -     breakpoint before starting.  */
>>> -  if (!have_minimal_symbols (current_program_space))
>>> +  /* Abort the start command if 'main' cannot be resolved.  */
>>> +  struct symbol *sym
>>> +    = lookup_symbol_search_name (main_name (), nullptr,
>>> +				 SEARCH_FUNCTION_DOMAIN).symbol;
>>> +
>>> +  if (sym == nullptr)
>>>      error (_("No symbol table loaded.  Use the \"file\" command."));
>>>
>>>    /* Run the program until reaching the main procedure...  */
>>
>> Tom Tromey (cc-ed) could confirm this, but this may regress Ada
>> debugging for the case where we don't have debug info available. My
>> understanding is that minimal symbols come from the ELF file, and
>> lookup_symbol_search_name with those parameters may not dive into
>> minimal symbols and so won't find what it is looking for.
>>
>> A test for this is trying to start a Ada program with debug info and
>> then trying to start one without debug info ("strip -g" the binary).
>>
>> The latter may not work anymore.
>>
> 
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