On Thu, 16 Mar 2017 11:19:44 -0700 Linus Torvalds <[email protected]> wrote:
> On Thu, Mar 16, 2017 at 11:09 AM, Steven Rostedt <[email protected]> wrote: > > + > > + /* Since we don't care about cs, move flags there to simplify > > return */ > > + movl 14*4(%esp), %eax > > + movl %eax, 13*4(%esp) > > + > > + /* Move return ip back to its original location */ > > + movl 12*4(%esp), %eax > > + movl %eax, 14*4(%esp) > > Could this perhaps be removed entirely? > > The return code could instead do: > > ... restore all the normal registers .. > > # Now restore flags that is under the return address and our > fake __KERNEL_CS > pushl 8(%esp) > popfl > > # and then return, skipping __KERNEL_CS and %flasg > ret $8 > > which is smaller and simpler than (again) playing games with stack entries. > > Linus The thing is we don't return, we jump to the location that may be modified to run the function graph tracer. .Lftrace_ret: #ifdef CONFIG_FUNCTION_GRAPH_TRACER .globl ftrace_graph_call ftrace_graph_call: jmp ftrace_stub <- this can turn to a jump to function graph tracing #endif WEAK(ftrace_stub) ret END(ftrace_caller) [...] popf /* flags is in the cs location */ jmp .Lftrace_ret -- Steve
