Commit f17c4e01 authored by Michael Ellerman's avatar Michael Ellerman
Browse files

powerpc/module: Mark module stubs with a magic value 



When a module is loaded, calls out to the kernel go via a stub which is
generated at runtime. One of these stubs is used to call _mcount(),
which is the default target of tracing calls generated by the compiler
with -pg.

If dynamic ftrace is enabled (which it typically is), another stub is
used to call ftrace_caller(), which is the target of tracing calls when
ftrace is actually active.

ftrace then wants to disable the calls to _mcount() at module startup,
and enable/disable the calls to ftrace_caller() when enabling/disabling
tracing - all of these it does by patching the code.

As part of that code patching, the ftrace code wants to confirm that the
branch it is about to modify, is in fact a call to a module stub which
calls _mcount() or ftrace_caller().

Currently it does that by inspecting the instructions and confirming
they are what it expects. Although that works, the code to do it is
pretty intricate because it requires lots of knowledge about the exact
format of the stub.

We can make that process easier by marking the generated stubs with a
magic value, and then looking for that magic value. Altough this is not
as rigorous as the current method, I believe it is sufficient in
practice.

Reviewed-by: default avatarBalbir Singh <bsingharora@gmail.com>
Reviewed-by: default avatarTorsten Duwe <duwe@suse.de>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
parent 136cd345

arch/powerpc/include/asm/module.h

+1 −2
Original line number Diff line number Diff line
@@ -78,8 +78,7 @@ struct mod_arch_specific { 
#    endif	/* MODULE */

#endif



bool is_module_trampoline(u32 *insns);

int module_trampoline_target(struct module *mod, u32 *trampoline,

int module_trampoline_target(struct module *mod, unsigned long trampoline,

			     unsigned long *target);



#ifdef CONFIG_DYNAMIC_FTRACE

arch/powerpc/kernel/ftrace.c

+4 −10
Original line number Diff line number Diff line
@@ -106,10 +106,9 @@ static int 
__ftrace_make_nop(struct module *mod,

		  struct dyn_ftrace *rec, unsigned long addr)

{

	unsigned int op;

	unsigned long entry, ptr;

	unsigned long entry, ptr, tramp;

	unsigned long ip = rec->ip;

	void *tramp;

	unsigned int op;



	/* read where this goes */

	if (probe_kernel_read(&op, (void *)ip, sizeof(int)))
@@ -122,14 +121,9 @@ __ftrace_make_nop(struct module *mod, 
	}



	/* lets find where the pointer goes */

	tramp = (void *)find_bl_target(ip, op);



	pr_devel("ip:%lx jumps to %p", ip, tramp);

	tramp = find_bl_target(ip, op);



	if (!is_module_trampoline(tramp)) {

		pr_err("Not a trampoline\n");

		return -EINVAL;

	}

	pr_devel("ip:%lx jumps to %lx", ip, tramp);



	if (module_trampoline_target(mod, tramp, &ptr)) {

		pr_err("Failed to get trampoline target\n");

arch/powerpc/kernel/module_64.c

+26 −52
Original line number Diff line number Diff line
@@ -96,6 +96,8 @@ static unsigned int local_entry_offset(const Elf64_Sym *sym) 
}

#endif



#define STUB_MAGIC 0x73747562 /* stub */



/* Like PPC32, we need little trampolines to do > 24-bit jumps (into

   the kernel itself).  But on PPC64, these need to be used for every

   jump, actually, to reset r2 (TOC+0x8000). */
@@ -105,7 +107,8 @@ struct ppc64_stub_entry 
	 * need 6 instructions on ABIv2 but we always allocate 7 so

	 * so we don't have to modify the trampoline load instruction. */

	u32 jump[7];

	u32 unused;

	/* Used by ftrace to identify stubs */

	u32 magic;

	/* Data for the above code */

	func_desc_t funcdata;

};
@@ -139,70 +142,39 @@ static u32 ppc64_stub_insns[] = { 
};



#ifdef CONFIG_DYNAMIC_FTRACE



static u32 ppc64_stub_mask[] = {

	0xffff0000,

	0xffff0000,

	0xffffffff,

	0xffffffff,

#if !defined(_CALL_ELF) || _CALL_ELF != 2

	0xffffffff,

#endif

	0xffffffff,

	0xffffffff

};



bool is_module_trampoline(u32 *p)

int module_trampoline_target(struct module *mod, unsigned long addr,

			     unsigned long *target)

{

	unsigned int i;

	u32 insns[ARRAY_SIZE(ppc64_stub_insns)];



	BUILD_BUG_ON(sizeof(ppc64_stub_insns) != sizeof(ppc64_stub_mask));

	struct ppc64_stub_entry *stub;

	func_desc_t funcdata;

	u32 magic;



	if (probe_kernel_read(insns, p, sizeof(insns)))

	if (!within_module_core(addr, mod)) {

		pr_err("%s: stub %lx not in module %s\n", __func__, addr, mod->name);

		return -EFAULT;

	}



	for (i = 0; i < ARRAY_SIZE(ppc64_stub_insns); i++) {

		u32 insna = insns[i];

		u32 insnb = ppc64_stub_insns[i];

		u32 mask = ppc64_stub_mask[i];

	stub = (struct ppc64_stub_entry *)addr;



		if ((insna & mask) != (insnb & mask))

			return false;

	if (probe_kernel_read(&magic, &stub->magic, sizeof(magic))) {

		pr_err("%s: fault reading magic for stub %lx for %s\n", __func__, addr, mod->name);

		return -EFAULT;

	}



	return true;

	if (magic != STUB_MAGIC) {

		pr_err("%s: bad magic for stub %lx for %s\n", __func__, addr, mod->name);

		return -EFAULT;

	}



int module_trampoline_target(struct module *mod, u32 *trampoline,

			     unsigned long *target)

{

	u32 buf[2];

	u16 upper, lower;

	long offset;

	void *toc_entry;



	if (probe_kernel_read(buf, trampoline, sizeof(buf)))

	if (probe_kernel_read(&funcdata, &stub->funcdata, sizeof(funcdata))) {

		pr_err("%s: fault reading funcdata for stub %lx for %s\n", __func__, addr, mod->name);

                return -EFAULT;

	}



	upper = buf[0] & 0xffff;

	lower = buf[1] & 0xffff;



	/* perform the addis/addi, both signed */

	offset = ((short)upper << 16) + (short)lower;



	/*

	 * Now get the address this trampoline jumps to. This

	 * is always 32 bytes into our trampoline stub.

	 */

	toc_entry = (void *)mod->arch.toc + offset + 32;



	if (probe_kernel_read(target, toc_entry, sizeof(*target)))

		return -EFAULT;

	*target = stub_func_addr(funcdata);



	return 0;

}



#endif



/* Count how many different 24-bit relocations (different symbol,
@@ -447,6 +419,8 @@ static inline int create_stub(const Elf64_Shdr *sechdrs, 
	entry->jump[0] |= PPC_HA(reladdr);

	entry->jump[1] |= PPC_LO(reladdr);

	entry->funcdata = func_desc(addr);

	entry->magic = STUB_MAGIC;



	return 1;

}

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