ftrace/x86_32: Set ftrace_stub to weak to prevent gcc from using short jumps to it

commit 847fa1a6 upstream.

With new binutils, gcc may get smart with its optimization and change a jmp
from a 5 byte jump to a 2 byte one even though it was jumping to a global
function. But that global function existed within a 2 byte radius, and gcc
was able to optimize it. Unfortunately, that jump was also being modified
when function graph tracing begins. Since ftrace expected that jump to be 5
bytes, but it was only two, it overwrote code after the jump, causing a
crash.

This was fixed for x86_64 with commit 8329e818, with the same subject as
this commit, but nothing was done for x86_32.

Fixes: d61f82d0 ("ftrace: use dynamic patching for updating mcount calls")
Reported-by: Colin Ian King <colin.king@canonical.com>
Tested-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
[bwh: Backported to 3.16: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/x86/kernel/entry_32.S

@@ -1086,8 +1086,8 @@ ftrace_graph_call:
 	jmp ftrace_stub
 #endif
 
-.globl ftrace_stub
-ftrace_stub:
+/* This is weak to keep gas from relaxing the jumps */
+WEAK(ftrace_stub)
 	ret
 END(ftrace_caller)