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学习gdb-从pr开始
Add detailed analysis and solution for frame base address issue in GDB on LoongArch architecture.
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‎posts/study-gdb-从pr开始1.md(draft)‎

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pr 地址 https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=a4b1ac7f1f3b5eb05d35226017f9b7cc8bc94ac3
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# 1 解析标题
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```
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gdb: LoongArch: Record correct frame base address
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```
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可以理解这是为了记录正确的栈基地址，说明在提交之前 gdb 反馈的栈基地址是错误的
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# 2 解析摘要
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## 问题描述
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```
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Description of Problem:
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The frame_base structure is defined in gdb/frame-base.h, a typical
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implementation of frame_base is return the same value for frame base,
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locals base and args base. When debugging following code on LoongArch,
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the outputs of locals base and args base are not equal to frame base
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address in frame base register. The frame base register is sp(r3) or
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fp(r22) on LoongArch. This problem only occurs when frame base register
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is sp, there is no problem when fp is used as frame base register. When
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using gcc option -fomit-frame-pointer or writing asm code without using
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fp, the frame base register is sp.
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```
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frame base address 栈帧基地址： 整个栈帧的关键地址，固定指向一点
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locals base address 局部变量基地址： gdb定位局部变量的基地址
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args base addrss 参数变量基地址：gdb定位参数变量的基地址
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LoongArch 的函数栈帧可以用两种寄存器定义“基地址”：
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帧指针 (fp)：寄存器 $r22
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栈指针 (sp)：寄存器 $r3
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此处可以知道基地址的数据结构在frame-bash文件中定义，且说明了gdb调试过程中每个函数均有对应的栈帧，存储着frame base（基地址）、locals base（当前地址）和args base （参数基地址）
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当在la机器上进行debug时，涉及一个简单文件，预想local base和arg base与frame base address 相同，但是在使用-fomit-frame-pointer 指定不使用fp作为基地址时，问题出现了，frame base与其他两者地址不同。
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## 测试代码及结果
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```
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$ cat test.c
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int main()
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{
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unsigned long a= 1, b = 1;
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a = 2;
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b = 2;
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return 0;
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}
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$ gcc -g -fomit-frame-pointer test.c -o test
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$ gdb test
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...
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(gdb) start
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...
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Temporary breakpoint 1, main () at test.c:4
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4 unsigned long a= 1, b = 1;
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(gdb) disas
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Dump of assembler code for function main:
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0x0000555555554740 <+0>: addi.d $sp, $sp, -16
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=> 0x0000555555554744 <+4>: li.w $t0, 1
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0x0000555555554748 <+8>: st.d $t0, $sp, 8
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0x000055555555474c <+12>: li.w $t0, 1
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0x0000555555554750 <+16>: stptr.d $t0, $sp, 0
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0x0000555555554754 <+20>: li.w $t0, 2
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0x0000555555554758 <+24>: st.d $t0, $sp, 8
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0x000055555555475c <+28>: li.w $t0, 2
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0x0000555555554760 <+32>: stptr.d $t0, $sp, 0
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0x0000555555554764 <+36>: move $t0, $zero
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0x0000555555554768 <+40>: move $a0, $t0
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0x000055555555476c <+44>: addi.d $sp, $sp, 16
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0x0000555555554770 <+48>: ret
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End of assembler dump.
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(gdb) p $sp
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$1 = (void *) 0x7ffffffeb130
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(gdb) info frame
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Stack level 0, frame at 0x7ffffffeb140:
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pc = 0x555555554744 in main (test.c:4); saved pc = 0x7ffff7e3d874
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source language c.
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Arglist at 0x7ffffffeb140, args:
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Locals at 0x7ffffffeb140, Previous frame's sp is 0x7ffffffeb140
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```
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通过上述栈帧可以看到：
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```
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$sp = 0x7ffffffeb130
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args base address = 0x7ffffffeb140
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locals base address = 0x7ffffffeb140
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```
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为了对照，我们再运行一组使用fp作为基地址的情况
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```
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(gdb) disas
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Dump of assembler code for function main:
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0x0000555555554758 <+0>: addi.d $sp, $sp, -16
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=> 0x000055555555475c <+4>: li.w $t0, 1
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0x0000555555554760 <+8>: st.d $t0, $sp, 8
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0x0000555555554764 <+12>: li.w $t0, 1
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0x0000555555554768 <+16>: stptr.d $t0, $sp, 0
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0x000055555555476c <+20>: li.w $t0, 2
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0x0000555555554770 <+24>: st.d $t0, $sp, 8
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0x0000555555554774 <+28>: li.w $t0, 2
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0x0000555555554778 <+32>: stptr.d $t0, $sp, 0
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0x000055555555477c <+36>: move $t0, $zero
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0x0000555555554780 <+40>: move $a0, $t0
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0x0000555555554784 <+44>: addi.d $sp, $sp, 16
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0x0000555555554788 <+48>: ret
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End of assembler dump.
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(gdb) p $sp
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$1 = (void *) 0x7ffffffeec40
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(gdb) info frame
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Stack level 0, frame at 0x7ffffffeec50:
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pc = 0x55555555475c in main (test.c:3); saved pc = 0x7ffff7e22208
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source language c.
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Arglist at 0x7ffffffeec50, args:
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Locals at 0x7ffffffeec50, Previous frame's sp is 0x7ffffffeec50
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```
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可以看到此时frame address == args address == locals address
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## 解决方案
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(3) Solution:
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Implement loongarch_frame_base structure and loongarch_frame_base_address()
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to record the correct frame base address stored in sp or fp register. It can
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be calculated by subtracting the framebase_offset from the prev_sp recorded
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in loongarch_frame_cache. And locals base and args base here are equal to
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frame base.
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不使用默认的frame_base_address函数，引入loongarch_frame_base 自定义的结构体以及loongarch_frame_base_address函数去记录正确的sp/fp地址。
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可以通过prev_sp（上一个栈帧的sp/fp）减去framebase_offset（偏移量）来修正
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具体代码：
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自己思考一下实现思路：
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首先不再使用默认的frame_base_address获取frame地址
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其次 查看默认的frame_base_address结构体可以看到：
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```
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struct frame_base {
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frame_unwind_func *unwind_func;
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frame_base_addr_func *frame_base_address;
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frame_base_addr_func *locals_base_address;
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frame_base_addr_func *args_base_address;
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};
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```
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首先，使用同一个默认函数，因此需要将他们拆分开来
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其次，需要实现一个获取frame地址的函数，计算方式是上一个栈帧的地址-offset，具体实现如下：
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```
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index f88fb5f0b09ef9439e039560041c5693f4f83815..e94b88bc010aebd408f1d6670fbb1e99fcf95895 100644 (file)
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--- a/gdb/loongarch-tdep.c
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+++ b/gdb/loongarch-tdep.c
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@@ -22,6 +22,7 @@
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#include "dwarf2/frame.h"
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#include "elf-bfd.h"
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#include "extract-store-integer.h"
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+#include "frame-base.h"
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#include "frame-unwind.h"
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#include "gdbcore.h"
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#include "linux-record.h"
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@@ -787,6 +788,26 @@ static const struct frame_unwind_legacy loongarch_frame_unwind (
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/*.prev_arch =*/nullptr
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);
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+/* Return the frame base address of *THIS_FRAME. */
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+
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+static CORE_ADDR
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+loongarch_frame_base_address (const frame_info_ptr &this_frame, void **this_cache)
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+{
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+ struct loongarch_frame_cache *cache
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+ = loongarch_frame_cache (this_frame, this_cache);
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+
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+ return cache->prev_sp - cache->framebase_offset;
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+}
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+
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+/* LoongArch default frame base information. */
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+static frame_base loongarch_frame_base =
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+{
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+ &loongarch_frame_unwind,
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+ loongarch_frame_base_address,
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+ loongarch_frame_base_address,
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+ loongarch_frame_base_address
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+};
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+
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/* Write the contents of buffer VAL into the general-purpose argument
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register defined by GAR in REGCACHE. GAR indicates the available
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general-purpose argument registers which should be a value in the
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@@ -2207,6 +2228,8 @@ loongarch_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
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dwarf2_append_unwinders (gdbarch);
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frame_unwind_append_unwinder (gdbarch, &loongarch_frame_unwind);
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+ frame_base_set_default (gdbarch, &loongarch_frame_base);
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+
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/* Hook in OS ABI-specific overrides, if they have been registered. */
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gdbarch_init_osabi (info, gdbarch);
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set_gdbarch_register_reggroup_p (gdbarch, loongarch_register_reggroup_p);
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```

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