linux-kernel - Re: [PATCH v2 4/4] kernel: add support for init

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Message-ID: <878uz544si.fsf@rustcorp.com.au>
Date:	Tue, 10 Sep 2013 15:05:57 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Frantisek Hrbata <fhrbata@...hat.com>
Cc:	Kyle McMartin <kyle@...radead.org>, linux-kernel@...r.kernel.org,
	jstancek@...hat.com, keescook@...omium.org,
	peter.oberparleiter@...ibm.com, linux-arch@...r.kernel.org,
	arnd@...db.de, mgahagan@...hat.com, agospoda@...hat.com,
	akpm@...ux-foundation.org
Subject: Re: [PATCH v2 4/4] kernel: add support for init_array constructors

Frantisek Hrbata <fhrbata@...hat.com> writes:
> On Mon, Sep 09, 2013 at 10:44:03AM +0930, Rusty Russell wrote:
>> Kyle McMartin <kyle@...radead.org> writes:
>> > On Fri, Sep 06, 2013 at 07:51:18PM +0200, Frantisek Hrbata wrote:
>> >> > > v2: - reuse mod->ctors for .init_array section for modules, because gcc uses
>> >> > >       .ctors or .init_array, but not both at the same time
>> >> > >
>> >> > > Signed-off-by: Frantisek Hrbata <fhrbata@...hat.com>
>> >> > 
>> >> > Might be nice to document which gcc version changed this, so people can
>> >> > choose whether to cherry-pick this change?
>> >> 
>> >> Thank you for pointing this out. As per gcc git this was introduced by commit
>> >> ef1da80 and released in 4.7 version.
>> >> 
>> >> $ git describe --contains ef1da80
>> >> gcc-4_7_0-release~4358
>> >> 
>> >> Do you want me to post v3 with this info included in the descrition?
>> >> 
>> >
>> > It actually depends on the combination of binutils/ld and gcc you use, not
>> > simply which gcc version you use. :/
>> 
>> Indeed, and seems it was binutils 20110507 which actually handled it
>> properly.
>> 
>> AFAICT it's theoretically possible to have .ctors and .init_array in a
>> module.  Unlikely, but the patch should check for both and refuse to
>> load the module in that case.  Otherwise weird things would happen.
>
> I'm not sure if coexistence of .ctors and .init_array sections should result in
> denial of module, but I for sure know nothing about this :). Could you maybe
> privide one example of the "weird thing"?

Well, if we have both ctors and init_array, and we only call the ctors,
part of the module will be uninitialized.

I was thinking about something like the following (based on your
previous patch).

Thoughts?
Rusty.

From: Frantisek Hrbata <fhrbata@...hat.com>
Subject: kernel: add support for init_array constructors

This adds the .init_array section as yet another section with constructors. This
is needed because gcc could add __gcov_init calls to .init_array or .ctors
section, depending on gcc (and binutils) version .

v2: - reuse mod->ctors for .init_array section for modules, because gcc uses
      .ctors or .init_array, but not both at the same time
v3: - fail to load if that does happen somehow.

Signed-off-by: Frantisek Hrbata <fhrbata@...hat.com>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 83e2c31..bc2121f 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -473,6 +473,7 @@
 #define KERNEL_CTORS()	. = ALIGN(8);			   \
 			VMLINUX_SYMBOL(__ctors_start) = .; \
 			*(.ctors)			   \
+			*(.init_array)			   \
 			VMLINUX_SYMBOL(__ctors_end) = .;
 #else
 #define KERNEL_CTORS()
diff --git a/kernel/module.c b/kernel/module.c
index dc58274..d3f5a58 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2738,7 +2738,7 @@ static int check_modinfo(struct module *mod, struct load_info *info, int flags)
 	return 0;
 }
 
-static void find_module_sections(struct module *mod, struct load_info *info)
+static int find_module_sections(struct module *mod, struct load_info *info)
 {
 	mod->kp = section_objs(info, "__param",
 			       sizeof(*mod->kp), &mod->num_kp);
@@ -2768,6 +2768,18 @@ static void find_module_sections(struct module *mod, struct load_info *info)
 #ifdef CONFIG_CONSTRUCTORS
 	mod->ctors = section_objs(info, ".ctors",
 				  sizeof(*mod->ctors), &mod->num_ctors);
+	if (!mod->ctors)
+		mod->ctors = section_objs(info, ".init_array",
+				sizeof(*mod->ctors), &mod->num_ctors);
+	else if (find_sec(info, ".init_array")) {
+		/*
+		 * This shouldn't happen with same compiler and binutils
+		 * building all parts of the module.
+		 */
+		printk(KERN_WARNING "%s: has both .ctors and .init_array.\n",
+		       mod->name);
+		return -EINVAL;
+	}
 #endif
 
 #ifdef CONFIG_TRACEPOINTS
@@ -2806,6 +2818,8 @@ static void find_module_sections(struct module *mod, struct load_info *info)
 
 	info->debug = section_objs(info, "__verbose",
 				   sizeof(*info->debug), &info->num_debug);
+
+	return 0;
 }
 
 static int move_module(struct module *mod, struct load_info *info)
@@ -3263,7 +3277,9 @@ static int load_module(struct load_info *info, const char __user *uargs,
 
 	/* Now we've got everything in the final locations, we can
 	 * find optional sections. */
-	find_module_sections(mod, info);
+	err = find_module_sections(mod, info);
+	if (err)
+		goto free_unload;
 
 	err = check_module_license_and_versions(mod);
 	if (err)
--
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