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Q&A Ghidra decompiler: c_str() used in phantom string?

Maybe Ghidra's decompiler can't handle C++ syntax that well. In that case one has to fallback to assembly in the listing view. Leading to the c_str() call is 00102b51 e8 8a f7 CALL ...

posted 6mo ago by nteodosio‭ · edited 6mo ago by nteodosio‭

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#2: Post edited by

nteodosio‭ · 2023-11-18T18:10:58Z (6 months ago)

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Maybe Ghidra's decompiler can't handle C++ syntax that well.
In that case one has to fallback to assembly in the listing view.
Leading to the `c_str()` call is
00102b51 e8 8a f7 CALL <EXTERNAL>::std::operator<<
ff ff
00102b56 48 89 c3 MOV RBX,len
00102b59 48 8d 85 LEA len=>local_448,[RBP + -0x440]
c0 fb ff ff
00102b60 48 89 c7 MOV RDI,len
00102b63 e8 f8 f6 CALL <EXTERNAL>::std::__cxx11::basic_string<>::c_str
ff ff
Therefore, **`local_448` is the string whose `c_str()` method is being called**. This is easy to figure in this case even without knowing the calling convention given that almost nothing else is being set up between the adjacent `<<` and the `c_str` calls.
But just to make sure I looked it up and that matches indeed the calling convention used by GNU GCC:
> The GNU g++ compiler treats `this` as the implied first parameter to any nonstatic member function and behaves in all other respects as if the cdecl convention is being used.
>
> — The Ghidra Book, The Definitive Guide, by Chris Eagle and Kara Nance
and
> For 64-bit x86 binaries, cdecl varies by operating system; on Linux, up to six arguments are placed in registers RDI, RSI, RDX, RCX, R8, and R9, in that order, and any additional arguments spill onto the stack.
>
> — Ibid.

Maybe Ghidra's decompiler can't handle C++ syntax that well.
In that case one has to fallback to assembly in the listing view.
Leading to the `c_str()` call is
00102b51 e8 8a f7 CALL <EXTERNAL>::std::operator<<
ff ff
00102b56 48 89 c3 MOV RBX,len
00102b59 48 8d 85 LEA len=>local_448,[RBP + -0x440]
c0 fb ff ff
00102b60 48 89 c7 MOV RDI,len
00102b63 e8 f8 f6 CALL <EXTERNAL>::std::__cxx11::basic_string<>::c_str
ff ff
Therefore, **`local_448` is the string whose `c_str()` method is being called**. This is easy to figure in this case even without knowing the calling convention given that almost nothing else is being set up between the adjacent `<<` and the `c_str` calls.
But just to make sure I looked it up and that matches indeed the calling convention used by GNU GCC:
> The GNU g++ compiler treats `this` as the implied first parameter to any nonstatic member function and behaves in all other respects as if the cdecl convention is being used.
>
> — The Ghidra Book, The Definitive Guide, by Chris Eagle and Kara Nance
and
> For 64-bit x86 binaries, cdecl varies by operating system; on Linux, up to six arguments are placed in registers RDI, RSI, RDX, RCX, R8, and R9, in that order, and any additional arguments spill onto the stack.
>
> — Ibid.
Also there is nothing special about `c_str`, other string methods such as `length` are likewise problematic in the decompiler view but can be likewise correctly analysed in the assembly view.

#1: Initial revision by

nteodosio‭ · 2023-11-18T18:07:19Z (6 months ago)

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Maybe Ghidra's decompiler can't handle C++ syntax that well.

In that case one has to fallback to assembly in the listing view.

Leading to the `c_str()` call is

        00102b51 e8 8a f7        CALL       <EXTERNAL>::std::operator<<
                 ff ff
        00102b56 48 89 c3        MOV        RBX,len
        00102b59 48 8d 85        LEA        len=>local_448,[RBP + -0x440]
                 c0 fb ff ff
        00102b60 48 89 c7        MOV        RDI,len
        00102b63 e8 f8 f6        CALL       <EXTERNAL>::std::__cxx11::basic_string<>::c_str
                 ff ff

Therefore, **`local_448` is the string whose `c_str()` method is being called**. This is easy to figure in this case even without knowing the calling convention given that almost nothing else is being set up between the adjacent `<<` and the `c_str` calls. 

But just to make sure I looked it up and that matches indeed the calling convention used by GNU GCC:

> The GNU g++ compiler treats `this` as the implied first parameter to any nonstatic member function and behaves in all other respects as if the cdecl convention is being used.
>
> — The Ghidra Book, The Definitive Guide, by Chris Eagle and Kara Nance

and

> For 64-bit x86 binaries, cdecl varies by operating system; on Linux, up to six arguments are placed in registers RDI, RSI, RDX, RCX, R8, and R9, in that order, and any additional arguments spill onto the stack.
>
> — Ibid.

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