Re: [PATCH v3 3/3] Aarch64: Fix segfault when casting dummy calls

[Alan Hayward <Alan dot Hayward at arm dot com>]

> On 30 Oct 2018, at 16:31, Pedro Alves <palves@redhat.com> wrote:
> 
> On 10/30/2018 11:13 AM, Alan Hayward wrote:
> 
>>> On 29 Oct 2018, at 18:13, Pedro Alves <palves@redhat.com> wrote:
>>> 
>>> On 10/29/2018 02:56 PM, Alan Hayward wrote:
>>> 
>>>> A-ha! Now I understand why I get two calls into _push_dummy_call.
>>>> 
>>>> So to answer your question, the TYPE_CODE_PTR->TYPE_CODE_INT is the malloc call.
>>>> 
>>>> Then the next call to _push_dummy_call has a return type of 0, as expected.
>>>> This doesn’t segfault because it goes into language_pass_by_reference which
>>>> routes to default_pass_by_reference. The same as the C shared library version.
>>>> 
>>>> 
>>>> I’ve updated the test so it does {c,c++}*{debug nodebug}.
>>>> I can also update it to do both shared lib and non shared lib too. That should
>>>> cover everything.
>>> But still, why do you see a difference between shared library and non-shared
>>> library?
>> 
>> In all cases the function type is the same.
>> 
>> The difference is because with c++ && shared library, the code ends up in 
>> gnuv3_pass_by_reference(), which means it’s using the GNU G++ Version 3 ABI,
>> whereas with any other options (non shared or c) it ends up in
>> default_pass_by_reference().
> 
> The function is the same, and should have been compiled using the calling
> convention irrespective of whether it is linked into the main program,
> or linked into the separate library.  Right?
> 
> So, either I'm missing something, or in one of the cases (shared
> vs non-shared), we're calling the function incorrectly (along
> with anything else that depends on call ABI), no?
> 
> What am I missing?
> 
> What does:
> 
> (gdb) show cp-abi 
> The currently selected C++ ABI is "auto" (currently "gnu-v3").
> 
> show for you, in the shared and non-shared cases?
> 
> /me pokes a bit.
> 
> OK, I see what it is.
> 
> You've compiled the _main_ .cc without debug info as well:
> 
> g++ -c condbreak-solib-main.cc -o condbreak-solib-main.o -fno-inline
> g++ -c condbreak-solib-lib.cc -o condbreak-solib-lib.o -fno-inline
> g++ condbreak-solib-main.o condbreak-solib-lib.o
> 
> And if you do that, the program ends up with no debug info at
> all, and so GDB has no clue that this is a C++ program:
> 
> (gdb) start
> Temporary breakpoint 1 at 0x4004c1
> Starting program: /tmp/a.out 
> 
> Temporary breakpoint 1, 0x00000000004004c1 in main ()
> (gdb) show language 
> The current source language is "auto; currently c".
> (gdb)
> 
> 
> If you compile (only) the main.cc with debug info, like this:
> 
> - g++ -c condbreak-solib-main.cc -o condbreak-solib-main.o -fno-inline
> + g++ -c condbreak-solib-main.cc -o condbreak-solib-main.o -fno-inline -g
> 
> then GDB will know that the program is a C++ program.  And you'd
> still be calling a cmp3 function that has no debug info,
> and should thus trigger the bug.
> 

Yes, I hit the bug this way.

Many thanks for looking through this.


> 
> 
> So when we call cmp3 with GDB's language set to C, we land
> in default_pass_by_reference:
> 
> (gdb) show language 
> The current source language is "auto; currently c".
> (gdb) p (int) cmp3(word)
> 
> Thread 1 "gdb" hit Breakpoint 3, default_pass_by_reference (type=0x1b5a230) at src/gdb/language.c:669
> 669       return 0;
> (top-gdb) c
> Continuing.
> 
> When the language is set to C++, we end up in gnuv3_pass_by_reference:
> 
> (gdb) set language c++ 
> (gdb) p (int) cmp3(word)
> 
> Thread 1 "gdb" hit Breakpoint 4, gnuv3_pass_by_reference (type=0x1b33290) at src/gdb/gnu-v3-abi.c:1255
> 1255      type = check_typedef (type);
> (top-gdb) 
> 
> And this is because language_pass_by_reference uses the
> current language, instead of the symbol's language (arguably a bug):
> 
> (top-gdb) bt
> #0  0x000000000065be91 in gnuv3_pass_by_reference(type*) (type=0x1b33290)
>     at src/gdb/gnu-v3-abi.c:1255
> #1  0x0000000000543e2a in cp_pass_by_reference(type*) (type=0x1b33290) at src/gdb/cp-abi.c:229
> #2  0x00000000006cc09b in language_pass_by_reference(type*) (type=0x1b33290)
>     at src/gdb/language.c:660
> #3  0x000000000045a27a in default_return_in_first_hidden_param_p(gdbarch*, type*) (gdbarch=0x1b316b0, type=0x1b33290)
>     at src/gdb/arch-utils.c:861
> #4  0x0000000000640a86 in gdbarch_return_in_first_hidden_param_p(gdbarch*, type*) (gdbarch=0x1b316b0, type=0x1b33290)
>     at src/gdb/gdbarch.c:2739
> #5  0x00000000006a1011 in call_function_by_hand_dummy(value*, type*, int, value**, void (*)(void*, int), void*) (function=0x1b44730, default_return_type=0x1b33290, nargs=1, args=0x7fffffffc128, dummy_dtor=0x0, dummy_dtor_data=0x0)
>     at src/gdb/infcall.c:881
> 
> 
> 
> So that's the real difference.  Shared vs non-shared is just
> a kind of a red herring.  If you don't have debug info for
> libstdc++, for example, then probably GDB won't know that the
> no-debug-info program is a C++ program either.
> 
> So please adjust your test to eliminate use of the shared
> library, and build just the cmp3 source file without
> debug info.

Will do.


> 
>> Looking at the doc for GNU G++ Version 3 ABI:
>> https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html
>> The library needs to be linked against libstdc++.so to use it.
>> 
>> A quick ldd shows only the c++ .so is linked against it.
> 
> That wouldn't make much sense.  The whole program is using the
> same compiler/call/mangling ABI, certainly, which is what
> matters here.
> 
> Thanks,
> Pedro Alves