2.72 GDB调试时某些符号出现其他中后缀

https://scz.617.cn/unix/202303022110.txt

Q:

GDB调试时某些符号出现其他中后缀，比如

.isra.124.constprop.226
icmp_echo.part.0
_PyObject_VectorcallTstate.lto_priv.12

nm查看内核调试符号时也会碰上类似的中后缀，这都什么鬼玩意儿？

A: ChatGPT 2023-03-02

.constprop.

A prefix or infix added to symbol names by the GCC compiler when the
function or variable is a candidate for constant propagation optimization.
It indicates that the function or variable may be safely replaced by its
constant value at compile time.

Suppose we have the following C code

--------------------------------------------------------------------------
int add(int a, int b) {
  int c = a + b;
  return c;
}

int main() {
  int x = 2;
  int y = 3;
  int z = add(x, y);
  return z;
}
--------------------------------------------------------------------------

When compiled with GCC and optimized with the "-O2" flag, the compiler
will perform constant propagation optimization on the "add" function,
because the variables "a" and "b" are constant values at the call site.
This means that the compiler can replace the variables "a" and "b" with
their constant values (2 and 3, respectively) at compile time, and then
compute the result of the "add" function (which is 5) as a constant value.

As a result of this optimization, the compiler will add the ".constprop"
symbol to the name of the "add" function, to indicate that it has
undergone constant propagation optimization. The resulting assembly code
might look something like this

--------------------------------------------------------------------------
add.constprop:
    movl $5, %eax
    ret

main:
    movl $2, %edi
    movl $3, %esi
    call add.constprop
    movl %eax, %ebx
    movl %ebx, %eax
    ret
--------------------------------------------------------------------------

Here, we can see that the "add" function has been replaced with a single
instruction that computes the result of adding the constant values 2 and 3,
and the resulting function is named "add.constprop". This is an example of
how the ".constprop" symbol is used to indicate that a function has
undergone constant propagation optimization.

A: ChatGPT 2023-03-02

.part.

The ".part." infix in symbol names is often used by the GCC compiler to
indicate that a particular object file or function has been split as part
of the Split DWARF feature. This feature is designed to reduce the size of
debug information generated by the compiler, by splitting the debug
information into separate files that can be loaded on-demand at debug time.

For example, suppose we have a program "foo" that is compiled with the
-gsplit-dwarf flag, which enables the Split DWARF feature. During
compilation, the GCC compiler will split the debug information into
separate files, and will add the ".part." infix to the symbol names of the
split functions and variables.

As an example, suppose we have a function "bar" in the "foo" program that
has been split as part of the Split DWARF feature. The symbol name for
this function might appear as "bar.part.0" in the object file generated by
the compiler. The ".part.0" infix in the symbol name indicates that this
function has been split into a separate object file, with debug
information stored in a separate ".dwo" file.

When the program is debugged using a debugger that supports the Split
DWARF feature, the debugger will load the separate ".dwo" file for the
relevant function or variable, in order to provide access to the debug
information for that symbol. This can help to reduce the overall size of
the executable or shared library, while still allowing for comprehensive
debugging information to be available when needed.

A: ChatGPT 2023-03-02

The ".lto_priv." infix in symbol names is often used by the GCC compiler
to indicate that a particular object file or function has been transformed
as part of the Link-Time Optimization (LTO) feature. This feature is
designed to improve code optimization and reduce executable size by
performing inter-procedural optimization across different compilation
units.

For example, suppose we have a program "foo" that is compiled with the
-flto flag, which enables the LTO feature. During compilation, the GCC
compiler will perform inter-procedural optimization across different
source files, and will add the ".lto_priv." infix to the symbol names of
the transformed functions and variables.

As an example, suppose we have a function "bar" in the "foo" program that
has been transformed as part of the LTO feature. The symbol name for this
function might appear as "bar.lto_priv.12" in the object file generated by
the compiler. The ".lto_priv.12" infix in the symbol name indicates that
this function has been transformed as part of the LTO feature, and that it
is associated with the 12th LTO object file.

When the program is linked using the linker that supports the LTO feature,
the linker will perform additional inter-procedural optimization across
different compilation units, using the LTO object files generated by the
compiler. This can help to improve code optimization and reduce executable
size, by allowing the compiler and linker to perform more comprehensive
optimization across the entire program.

A: stackoverflow 2013-02-10

What does the GCC function suffix .constprop mean - [2013-02-10]
https://stackoverflow.com/questions/14796686/what-does-the-gcc-function-suffix-constprop-mean

They indicate functions which have been cloned during optimization. This
file implements constant propagation and merging. It looks for instructions
involving only constant operands and replaces them with a constant value
instead of an instruction.

If you have a static function and you happen to call it with a certain
constant (or constants) for one or some of its parameter(s), gcc can create
an optimized version just for that combination of parameter value(s).

A: Tomaz Solc 2020-02-14

Printing .lto_priv symbols in GDB - Tomaz Solc [2020-02-14]
https://www.tablix.org/~avian/blog/archives/2020/02/printing_lto_priv_symbols_in_gdb/

You have to quote the names of some variables when debugging a binary that
was compiled with link time optimization. You have to put it in quotes so
that gdb doesn't try to interpret the dot as an operator.

D: scz 2023-03-02

用单引号包裹符号，不要用双引号。有个不确定的经验，出现中缀时，可用单引号包
裹符号；只有后缀时，即使用单引号包裹符号也无法完成解析，只能删除后缀直接用
原始符号，此情况似乎并无其他变体。一些示例

x/i 'some.lto_priv.2'
x/i 'some.lto_priv.2'+134
info symbol 'some.lto_priv.2'+134

注意，单引号只能包裹带中缀的符号变体，不能包裹偏移量。

IDA对带这些带中后缀的符号有自己的处理方式，比如将
_PyObject_VectorcallTstate.lto_priv.12显示成_PyObject_VectorcallTstate_15，
在IDA中看不出前者，只能在GDB中看到前者，这可能产生误导。

关于GDB命令行补齐，参看

--------------------------------------------------------------------------
3.3 Command Completion
https://sourceware.org/gdb/onlinedocs/gdb/Completion.html

16 Examining the Symbol Table
https://sourceware.org/gdb/onlinedocs/gdb/Symbols.html
--------------------------------------------------------------------------

在GDB提示符下连按两次TAB完成命令行补齐，若有多个可选项，会显示它们，若只有
一个可选项，将自动补齐。另一种方式，按"M-?"，将显示所有可选项，无论有几个，
并不自动补齐，这是GDB官方文档的说法。

"M-?"是什么意思？

M是Meta的意思，"M-?"就是同时按住Meta和问号键。Windows没有Meta键，此时可以
先按ESC，松开后再按"Shift-?"，实测有效。用"M-?"时无需先按一次TAB。

可以用单引号将符号包起来，但不要用双引号，后者有幺蛾子，达不到预期目的。