powerpc/module: Create a special stub for ftrace_caller()

In order to support the new -mprofile-kernel ABI, we need to be able to
call from the module back to ftrace_caller() (in the kernel) without
using the module's r2. That is because the function in this module which
is calling ftrace_caller() may not have setup r2, if it doesn't
otherwise need it (ie. it accesses no globals).

To make that work we add a new stub which is used for calling
ftrace_caller(), which uses the kernel toc instead of the module toc.

Reviewed-by: Balbir Singh <bsingharora@gmail.com>
Reviewed-by: Torsten Duwe <duwe@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index 9629966e614be6aebf37e4c7427c430da6c15c5f..76c0963572f5c1c47b3e2f938bf7dca86f9c9628 100644 (file)
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -31,6 +31,7 @@
 #include <asm/code-patching.h>
 #include <linux/sort.h>
 #include <asm/setup.h>
+#include <asm/sections.h>
 
 /* FIXME: We don't do .init separately.  To do this, we'd need to have
    a separate r2 value in the init and core section, and stub between
@@ -671,10 +672,76 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
 }
 
 #ifdef CONFIG_DYNAMIC_FTRACE
+
+#ifdef CC_USING_MPROFILE_KERNEL
+
+#define PACATOC offsetof(struct paca_struct, kernel_toc)
+
+/*
+ * For mprofile-kernel we use a special stub for ftrace_caller() because we
+ * can't rely on r2 containing this module's TOC when we enter the stub.
+ *
+ * That can happen if the function calling us didn't need to use the toc. In
+ * that case it won't have setup r2, and the r2 value will be either the
+ * kernel's toc, or possibly another modules toc.
+ *
+ * To deal with that this stub uses the kernel toc, which is always accessible
+ * via the paca (in r13). The target (ftrace_caller()) is responsible for
+ * saving and restoring the toc before returning.
+ */
+static unsigned long create_ftrace_stub(const Elf64_Shdr *sechdrs, struct module *me)
+{
+       struct ppc64_stub_entry *entry;
+       unsigned int i, num_stubs;
+       static u32 stub_insns[] = {
+               0xe98d0000 | PACATOC,   /* ld      r12,PACATOC(r13)     */
+               0x3d8c0000,             /* addis   r12,r12,<high>       */
+               0x398c0000,             /* addi    r12,r12,<low>        */
+               0x7d8903a6,             /* mtctr   r12                  */
+               0x4e800420,             /* bctr                         */
+       };
+       long reladdr;
+
+       num_stubs = sechdrs[me->arch.stubs_section].sh_size / sizeof(*entry);
+
+       /* Find the next available stub entry */
+       entry = (void *)sechdrs[me->arch.stubs_section].sh_addr;
+       for (i = 0; i < num_stubs && stub_func_addr(entry->funcdata); i++, entry++);
+
+       if (i >= num_stubs) {
+               pr_err("%s: Unable to find a free slot for ftrace stub.\n", me->name);
+               return 0;
+       }
+
+       memcpy(entry->jump, stub_insns, sizeof(stub_insns));
+
+       /* Stub uses address relative to kernel toc (from the paca) */
+       reladdr = (unsigned long)ftrace_caller - kernel_toc_addr();
+       if (reladdr > 0x7FFFFFFF || reladdr < -(0x80000000L)) {
+               pr_err("%s: Address of ftrace_caller out of range of kernel_toc.\n", me->name);
+               return 0;
+       }
+
+       entry->jump[1] |= PPC_HA(reladdr);
+       entry->jump[2] |= PPC_LO(reladdr);
+
+       /* Eventhough we don't use funcdata in the stub, it's needed elsewhere. */
+       entry->funcdata = func_desc((unsigned long)ftrace_caller);
+       entry->magic = STUB_MAGIC;
+
+       return (unsigned long)entry;
+}
+#else
+static unsigned long create_ftrace_stub(const Elf64_Shdr *sechdrs, struct module *me)
+{
+       return stub_for_addr(sechdrs, (unsigned long)ftrace_caller, me);
+}
+#endif
+
 int module_finalize_ftrace(struct module *mod, const Elf_Shdr *sechdrs)
 {
        mod->arch.toc = my_r2(sechdrs, mod);
-       mod->arch.tramp = stub_for_addr(sechdrs, (unsigned long)ftrace_caller, mod);
+       mod->arch.tramp = create_ftrace_stub(sechdrs, mod);
 
        if (!mod->arch.tramp)
                return -ENOENT;