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+In this lesson we would talk about how to debug our UEFI application and drivers with GDB.
+
+To perfrom debug with GDB we need to load debug symbols at the correct offset, for it we need to know:
+- app offset in memory
+- internal app sections offsets
+
+# Getting application offset in memory
+
+Run your QEMU with debug image of OVMF and log functionality enabled:
+```
+qemu-system-x86_64 \
+ -drive if=pflash,format=raw,readonly,file=Build/OvmfX64/DEBUG_GCC5/FV/OVMF.fd \
+ -drive format=raw,file=fat:rw:~/UEFI_disk \
+ -net none \
+ -nographic \
+ -global isa-debugcon.iobase=0x402 \
+ -debugcon file:debug.log
+```
+In a separate terminal window execute:
+```
+tail -f debug.log
+```
+This way you can follow log runtime.
+
+Wait for UEFI shell boot and execute our `ShowBootVariables.efi` application. After you do it, something similar to these messages would be produced to `debug.log`:
+```
+Loading driver at 0x00006649000 EntryPoint=0x0000664C80F ShowBootVariables.efi
+InstallProtocolInterface: BC62157E-3E33-4FEC-9920-2D3B36D750DF 666B898
+ProtectUefiImageCommon - 0x666B4C0
+ - 0x0000000006649000 - 0x0000000000005540
+InstallProtocolInterface: 752F3136-4E16-4FDC-A22A-E5F46812F4CA 7EA36C8
+```
+
+Right now we are interested in this string:
+```
+Loading driver at 0x00006649000 EntryPoint=0x0000664C80F ShowBootVariables.efi
+```
+Our image is loaded to `0x6649000`.
+
+# Getting application sections offset
+
+Use `objdump` to see what sections are inside `.efi` executable:
+```
+$ objdump -h Build/UefiLessonsPkg/DEBUG_GCC5/X64/ShowBootVariables.efi
+
+Build/UefiLessonsPkg/DEBUG_GCC5/X64/ShowBootVariables.efi: file format pei-x86-64
+
+Sections:
+Idx Name Size VMA LMA File off Algn
+ 0 .text 00004d00 0000000000000240 0000000000000240 00000240 2**4
+ CONTENTS, ALLOC, LOAD, READONLY, CODE
+ 1 .data 00000580 0000000000004f40 0000000000004f40 00004f40 2**4
+ CONTENTS, ALLOC, LOAD, DATA
+ 2 .reloc 00000080 00000000000054c0 00000000000054c0 000054c0 2**2
+ CONTENTS, ALLOC, LOAD, READONLY, DATA
+```
+And what sections are inside `.debug` version:
+```
+$ objdump -h Build/UefiLessonsPkg/DEBUG_GCC5/X64/ShowBootVariables.debug
+
+Build/UefiLessonsPkg/DEBUG_GCC5/X64/ShowBootVariables.debug: file format elf64-x86-64
+
+Sections:
+Idx Name Size VMA LMA File off Algn
+ 0 .text 00004cf5 0000000000000240 0000000000000240 000000c0 2**6
+ CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
+ 1 .data 000004a1 0000000000004f40 0000000000004f40 00004dc0 2**6
+ CONTENTS, ALLOC, LOAD, RELOC, DATA
+ 2 .eh_frame 00000000 0000000000005400 0000000000005400 00005280 2**3
+ CONTENTS, ALLOC, LOAD, READONLY, DATA
+ 3 .rela 00000510 0000000000005400 0000000000005400 00005280 2**3
+ CONTENTS, READONLY
+ 4 .build-id 00000024 0000000000005910 0000000000005910 00005790 2**2
+ CONTENTS, READONLY
+ 5 .debug_info 000283a4 0000000000000000 0000000000000000 000057b4 2**0
+ CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
+ 6 .debug_abbrev 00003635 0000000000000000 0000000000000000 0002db58 2**0
+ CONTENTS, READONLY, DEBUGGING, OCTETS
+ 7 .debug_loc 00009fc9 0000000000000000 0000000000000000 0003118d 2**0
+ CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
+ 8 .debug_aranges 00000570 0000000000000000 0000000000000000 0003b156 2**0
+ CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
+ 9 .debug_ranges 000013f0 0000000000000000 0000000000000000 0003b6c6 2**0
+ CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
+ 10 .debug_line 00007a5b 0000000000000000 0000000000000000 0003cab6 2**0
+ CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
+ 11 .debug_str 000069e4 0000000000000000 0000000000000000 00044511 2**0
+ CONTENTS, READONLY, DEBUGGING, OCTETS
+ 12 .debug_frame 000015a8 0000000000000000 0000000000000000 0004aef8 2**3
+ CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
+```
+As you can see `*.efi` file is a version without any `.debug*` sections.
+
+Right now we are interested in `File off` column of the image that is getting executed - `ShowBootVariables.efi`.
+
+- `.text` section has offset `0x240`
+- `.data` section has offset `0x4f40`
+
+# Loading debug symbols to GDB
+
+First of all, to make remote debugging possible we have to provide `-s` parameter for QEMU. This is a shorthand for `tcp::1234`. QEMU will listen for GDB connections on this port.
+```
+qemu-system-x86_64 \
+ -drive if=pflash,format=raw,readonly,file=Build/OvmfX64/DEBUG_GCC5/FV/OVMF.fd \
+ -drive format=raw,file=fat:rw:~/UEFI_disk \
+ -net none \
+ -nographic \
+ -global isa-debugcon.iobase=0x402 \
+ -debugcon file:debug.log \
+ -s
+```
+
+Now we need to calculate addresses for `.text` and `.data` sections:
+```
+0x6649000 + 0x240 = 0x6649240
+0x6649000 + 0x4f40 = 0x664df40
+```
+
+Run GDB and load symbols into it:
+```
+$ gdb
+(gdb) add-symbol-file Build/UefiLessonsPkg/DEBUG_GCC5/X64/ShowBootVariables.debug 0x6649240 -s .data 0x664df40
+add symbol table from file "Build/UefiLessonsPkg/DEBUG_GCC5/X64/ShowBootVariables.debug" at
+ .text_addr = 0x6649240
+ .data_addr = 0x664df40
+(y or n) y
+Reading symbols from Build/UefiLessonsPkg/DEBUG_GCC5/X64/ShowBootVariables.debug...
+```
+
+The entry point of our application is `ShellAppMain` function. In a separate terminal find the string in a file where it is defined:
+```
+$ grep -n ShellAppMain UefiLessonsPkg/ShowBootVariables/ShowBootVariables.c
+67:INTN EFIAPI ShellAppMain(IN UINTN Argc, IN CHAR16 **Argv)
+```
+
+Set breakpint at that place:
+```
+(gdb) b UefiLessonsPkg/ShowBootVariables/ShowBootVariables.c:67
+Breakpoint 1 at 0x664c50f: file /home/kostr/tiano/edk2/UefiLessonsPkg/ShowBootVariables/ShowBootVariables.c, line 68.
+```
+
+Connect to the remote:
+```
+(gdb) target remote :1234
+Remote debugging using :1234
+warning: No executable has been specified and target does not support
+determining executable automatically. Try using the "file" command.
+0x000000000717d841 in ?? ()
+```
+
+This will stop QEMU, so we need to execute `c` (=continue) to make QEMU interactive again:
+```
+(gdb) c
+Continuing.
+```
+Now GDB is waiting for a breakpoint hit.
+
+Swith to QEMU terminal and run our app
+```
+FS0:\> ShowBootVariables.efi
+```
+On GDB you should receive a breakpoint hit:
+```
+Breakpoint 1, ShellAppMain (Argc=1, Argv=0x666a398) at /home/kostr/tiano/edk2/UefiLessonsPkg/ShowBootVariables/ShowBootVariables.c:73
+73 Status = GetNvramVariable(L"BootCurrent", &gEfiGlobalVariableGuid, (VOID**)&BootCurrent, &OptionSize);
+(gdb)
+```
+
+![gdb](gdb.png?raw=true "gdb")
+
+To ease GDB development I suggest you to try TUI mode (https://sourceware.org/gdb/onlinedocs/gdb/TUI.html):
+```
+(gdb) tui enable
+```
+In this mode source code file would be displayed right above the GDB prompt.
+
+![gdb_tui](gdb_tui.png?raw=true "gdb_tui")
+
+
+
+
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