4.3. User-Space Stack Backtraces

⁠4.3. User-Space Stack Backtraces

The probe point (pp) function indicates which particular event triggered the SystemTap event handler. A probe on the entry into a function would list the function name. However, in many cases the same probe point event may be triggered by many different modules in the program; this is particularly true for functions in shared libraries. A SystemTap backtrace of the user-space stack can provide additional context on how the probe point event is triggered.

The user-space stack backtrace generation is complicated by the compiler producing code optimized to eliminate stack frame pointers. However, the compiler also includes information in the debug information section to allow debugging tools to produce stack backtraces. SystemTap user-space stack backtrace mechanism makes use of that debug information to walk the stack to generate stack traces for 32-bit and 64-bit x86 processors; other processor architectures do not yet support the use of debug information to unwind the user-space stack. To ensure that the needed debug information is used to produce the user-space stack backtraces, use the -d executable option for executables and --ldd for shared libraries.

For example, you can use the user-space backtrack functions to see how the xmalloc function is being called by the ls command. With the debuginfo for the ls command installed, the following SystemTap command provides a backtrace each time the xmalloc function is called:

stap -d /bin/ls --ldd \
-e 'probe process("ls").function("xmalloc") {print_usyms(ubacktrace())}' \
-c "ls /"

When executed, this command produces output similar to the following:

bin	dev   lib	  media  net	     proc	sbin	 sys  var
boot	etc   lib64	  misc	 op_session  profilerc	selinux  tmp
cgroup	home  lost+found  mnt	 opt	     root	srv	 usr
 0x4116c0 : xmalloc+0x0/0x20 [/bin/ls]
 0x4116fc : xmemdup+0x1c/0x40 [/bin/ls]
 0x40e68b : clone_quoting_options+0x3b/0x50 [/bin/ls]
 0x4087e4 : main+0x3b4/0x1900 [/bin/ls]
 0x3fa441ec5d : __libc_start_main+0xfd/0x1d0 [/lib64/libc-2.12.so]
 0x402799 : _start+0x29/0x2c [/bin/ls]
 0x4116c0 : xmalloc+0x0/0x20 [/bin/ls]
 0x4116fc : xmemdup+0x1c/0x40 [/bin/ls]
 0x40e68b : clone_quoting_options+0x3b/0x50 [/bin/ls]
 0x40884a : main+0x41a/0x1900 [/bin/ls]
 0x3fa441ec5d : __libc_start_main+0xfd/0x1d0 [/lib64/libc-2.12.so]
 ...

For more details on the functions available for user-space stack backtraces, refer to ucontext-symbols.stp and ucontext-unwind.stp tapsets. You can also find the description of the functions in the aforementioned tapsets in the SystemTap Tapset Reference Manual.