通常,為了讓人們方便調試,會讓可執行文件儘可能小。在沒有調試信息的情況下獲取跟蹤信息,會大大降低其實用性。例如,在沒有調試信息的情況下,gdb包 會話的跟蹤信息可能如下所示:
[...] Backtrace was generated from '/usr/bin/epiphany' (no debugging symbols found) Using host libthread_db library "/lib/libthread_db.so.1". (no debugging symbols found) [Thread debugging using libthread_db enabled] [New Thread -1241265952 (LWP 12630)] (no debugging symbols found) 0xb7f25410 in __kernel_vsyscall () #0 0xb7f25410 in __kernel_vsyscall () #1 0xb741b45b in ?? () from /lib/libpthread.so.0 [...]
??
會顯示調試信息缺失的位置,以及調用該函數的庫或可執行文件的名稱。同樣,當出現 (no debugging symbols found)
時,應查找所述文件名。
要獲得對程序開發人員有用的正確跟蹤,請遵循以下章節。大多數 Arch 官方軟體包都有單獨的調試文件,可以通過 Debuginfod 下載(參見 #獲取跟蹤 )。 如果沒有在可執行文件中添加enhanced debugging information,則必須啟用調試符號 重建軟體包 。
使用完整的堆棧跟蹤來告知開發人員您之前發現的錯誤。
獲取追蹤文件
實際的回溯(或堆棧跟蹤)可以通過 gdb包 (GNU 調試器)獲取。根據是否應啟動程序的新實例、是否應附加到現有進程、或是否應檢查之前的崩潰,可以以多種方式使用調試器。
啟動程序的新實例
Start gdb with an executable program that can be found in $PATH
(or a full path):
$ gdb application
gdb automatically tries to download debug information and symbols for packages in the official repositories. When gdb asks whether Debuginfod should be enabled in the debugging session, answer y
:
This GDB supports auto-downloading debuginfo from the following URLs: <https://debuginfod.archlinux.org> Enable debuginfod for this session? (y or [n]) y Debuginfod has been enabled. To make this setting permanent, add 'set debuginfod enabled on' to .gdbinit. Downloading separate debug info for /usr/bin/application Reading symbols from /home/user/.cache/debuginfod_client/fbaee841e2ed2c11ecbbda26f39eeec1da23d6c3/debuginfo...
Then, within gdb, type run
followed by any arguments you wish the program to start with:
(gdb) run arguments
gdb --args application arguments...
and then use only run
without arguments within gdb. For example, to debug an application written in Python, run gdb --args /usr/bin/python /path/to/application
.Now do whatever is necessary to evoke the bug. gdb will automatically halt the application when it crashes and prompt for commands. In case of freezes or similar issues, press Ctrl+c
and you will be returned to the command prompt, too.
Then enable logging to a file:
(gdb) set logging enabled on
gdb.txt
. An alternate file name can be specified with set logging file trace.log
within gdb.And finally have the backtrace written to the specified file in the current working directory:
(gdb) thread apply all backtrace full
Attaching to an existing process
If the program you want to debug is already running, you need to first find its process ID. Tools such as pidof(1) or pgrep(1) are available. For example:
$ pidof python3
907171 491909
When the output does not give a unique ID, you can try more filtering or look at the output of ps aux
or pstree --show-pids
.
Attaching as regular user does not work by default due to restricted ptrace scope. The restriction can be lowered temporarily with echo 0 > /proc/sys/kernel/yama/ptrace_scope
or you can run gdb as a privileged user, e.g. using sudo.
Start gdb attaching it to the process:
$ gdb --pid=PID
gdb will ask if Debuginfod should be enabled in this debugging session, to which you should answer y
.
Note that attaching to a process has stopped it and it needs to be explicitly continued. This replaces the run
command from the workflow in the #Starting a new instance of a program section:
(gdb) continue
Now do whatever is necessary to evoke the bug in the attached process. Then proceed with enabling logging and obtaining the trace same as in #Starting a new instance of a program.
Examining a previous crash
To debug an application that has already crashed, you will want to invoke gdb on its core dump. See Core dump#Analyzing a core dump for details.
If debugging information for the crashed program is not available and a proper backtrace was not obtained, you may need to rebuild the package and reproduce the crash again.
Manually getting debug info
The first thing to do is to obtain the names of the packages which require rebuilding or the install of a debug package.
[...] Backtrace was generated from '/usr/bin/epiphany' (no debugging symbols found) Using host libthread_db library "/lib/libthread_db.so.1". (no debugging symbols found) [...]
For example for the above extract from a trace, the package name for the associated package can be obtained with pacman:
$ pacman -Qo /lib/libthread_db.so.1
/lib/libthread_db.so.1 is owned by glibc 2.5-8
The package is called glibc包 in version 2.5-8. Repeat this step for every package that needs debugging information.
Installing debug packages
A few mirrors currently distribute debug packages in accessible repositories. These are sponsored mirrors controlled by Arch Linux and are given access to the debug repositories.
- https://geo.mirror.pkgbuild.com (GeoDNS mirror)
To install a package you can install it directly from the repository. For example:
# pacman -U https://geo.mirror.pkgbuild.com/core-debug/os/x86_64/zstd-debug-1.5.2-2-x86_64.pkg.tar.zst
debug
mirror.Another option is to add the repositories to your pacman configuration.
/etc/pacman.conf
# Testing Repositories [core-testing-debug] Include = /etc/pacman.d/mirrorlist [extra-testing-debug] Include = /etc/pacman.d/mirrorlist [multilib-testing-debug] Include = /etc/pacman.d/mirrorlist # Stable repositories [core-debug] Include = /etc/pacman.d/mirrorlist [extra-debug] Include = /etc/pacman.d/mirrorlist [multilib-debug] Include = /etc/pacman.d/mirrorlist
Place a mirror with debug packages as the first one in the mirrorlist file:
/etc/pacman.d/mirrorlist
Server = https://geo.mirror.pkgbuild.com/$repo/os/$arch ...
Rebuilding packages
If debug information is not exposed through debuginfod (for example, when the package originates from the AUR), then it can be rebuilt from source. See ABS for packages in the official repositories, or AUR#Acquire build files for packages in the AUR.
To set the required #Compilation options, you can modify the makepkg configuration if you will only use makepkg for debug purposes. In other cases, you should modify package's PKGBUILD
file only for each package you would like to rebuild.
Compilation options
As of pacman 4.1, makepkg.conf(5) has debug compilation flags in DEBUG_CFLAGS
and DEBUG_CXXFLAGS
. To use them, enable the debug
makepkg option, and disable strip
.
These settings will force compilation with debug symbols and will disable their stripping from executables.
/etc/makepkg.conf
OPTIONS+=(debug !strip)
To apply this setting to a single package, modify the PKGBUILD
:
PKGBUILD
options=(debug !strip)
Alternatively you can put the debug information in a separate package by enabling both debug
and strip
, debug symbols will then be stripped from the main package and placed, together with source files to aid in stepping through the debugger, in a separate pkgbase-debug
package. This is advantageous if the package contains very large binaries (e.g. over a GB with debug symbols included) as it might cause freezing and other strange, unwanted behavior occurring.
/usr/lib/debug/
, and source files are installed under /usr/src/debug
. See the GDB documentation for more information about debug packages.glibc
Certain packages such as glibc are stripped regardless. Check the PKGBUILD
for sections such as:
strip $STRIP_BINARIES usr/bin/{gencat,getconf,getent,iconv,iconvconfig} \ usr/bin/{ldconfig,locale,localedef,makedb} \ usr/bin/{pcprofiledump,pldd,rpcgen,sln,sprof} \ usr/lib/getconf/* strip $STRIP_STATIC usr/lib/*.a strip $STRIP_SHARED usr/lib/{libanl,libBrokenLocale,libcidn,libcrypt}-*.so \ usr/lib/libnss_{compat,db,dns,files,hesiod,nis,nisplus}-*.so \ usr/lib/{libdl,libm,libnsl,libresolv,librt,libutil}-*.so \ usr/lib/{libmemusage,libpcprofile,libSegFault}.so \ usr/lib/{audit,gconv}/*.so
And remove them where appropriate.
Clang
Packages using Clang as the compiler will not build with the debug
option due to the debug flag -fvar-tracking-assignments'
not being handled (e.g. the previous js78 PKGBUILD).
Add the following at the top of the build()
function to only remove the flag for the affected package:
build() { CFLAGS=${CFLAGS/-fvar-tracking-assignments} CXXFLAGS=${CXXFLAGS/-fvar-tracking-assignments} [...]
LTO
Using Link-time optimization (LTO) will, both during compiling and in a debugger, use more memory[1][2]. Depending on the application, especially if it is a large one like Firefox or Qt, it might exceed the available memory. Build the application without LTO if this happens.
All packages in the official repositories are generally built with LTO.
Building and installing the package
Build the package from source using makepkg
while in the PKGBUILD
's directory. This could take some time:
$ makepkg
Then install the built package:
# pacman -U glibc-2.26-1-x86_64.pkg.tar.gz