Re: [PATCH 2/2] lib/vsprintf: reduce space taken by no_hash_pointers warning

Petr Mladek
2021-03-08T17:24:28Z

On Mon 2021-03-08 13:22:40, Geert Uytterhoeven wrote:
> Hi Petr,
>
> On Mon, Mar 8, 2021 at 11:16 AM Petr Mladek <pmladek@suse.com> wrote:
> > On Fri 2021-03-05 20:42:06, Marco Elver wrote:
> > > Move the no_hash_pointers warning string into __initconst section, so
> > > that it is discarded after init. Remove common start/end characters.
> > > Also remove repeated lines from the array, since the compiler can't
> > > remove duplicate strings for us since the array must appear in
> > > __initconst as defined.
> > >
> > > Note, a similar message appears in kernel/trace/trace.c, but compiling
> > > the feature is guarded by CONFIG_TRACING. It is not immediately obvious
> > > if a space-concious kernel would prefer CONFIG_TRACING=n. Therefore, it
> > > makes sense to keep the message for no_hash_pointers as __initconst, and
> > > not move the NOTICE-printing to a common function.
> > >
> > > Link: https://lkml.kernel.org/r/CAMuHMdULKZCJevVJcp7TxzLdWLjsQPhE8hqxhnztNi9bjT_cEw@mail.gmail.com
> > > Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
> > > Signed-off-by: Marco Elver <elver@google.com>
> > > ---
> > > lib/vsprintf.c | 30 +++++++++++++++++-------------
> > > 1 file changed, 17 insertions(+), 13 deletions(-)
> > >
> > > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > > index 4a14889ccb35..1095689c9c97 100644
> > > --- a/lib/vsprintf.c
> > > +++ b/lib/vsprintf.c
> > > @@ -2094,26 +2094,30 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
> > > bool no_hash_pointers __ro_after_init;
> > > EXPORT_SYMBOL_GPL(no_hash_pointers);
> > >
> > > +static const char no_hash_pointers_warning[8][55] __initconst = {
> > > + "******************************************************",
> > > + " NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE ",
> > > + " This system shows unhashed kernel memory addresses ",
> > > + " via the console, logs, and other interfaces. This ",
> > > + " might reduce the security of your system. ",
> > > + " If you see this message and you are not debugging ",
> > > + " the kernel, report this immediately to your system ",
> > > + " administrator! ",
> > > +};
> > > +
> > > static int __init no_hash_pointers_enable(char *str)
> > > {
> > > + /* Indices into no_hash_pointers_warning; -1 is an empty line. */
> > > + const int lines[] = { 0, 1, -1, 2, 3, 4, -1, 5, 6, 7, -1, 1, 0 };
> > > + int i;
> > > +
> > > if (no_hash_pointers)
> > > return 0;
> > >
> > > no_hash_pointers = true;
> > >
> > > - pr_warn("**********************************************************\n");
> > > - pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
> > > - pr_warn("** **\n");
> > > - pr_warn("** This system shows unhashed kernel memory addresses **\n");
> > > - pr_warn("** via the console, logs, and other interfaces. This **\n");
> > > - pr_warn("** might reduce the security of your system. **\n");
> > > - pr_warn("** **\n");
> > > - pr_warn("** If you see this message and you are not debugging **\n");
> > > - pr_warn("** the kernel, report this immediately to your system **\n");
> > > - pr_warn("** administrator! **\n");
> > > - pr_warn("** **\n");
> > > - pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
> > > - pr_warn("**********************************************************\n");
> > > + for (i = 0; i < ARRAY_SIZE(lines); i++)
> > > + pr_warn("**%54s**\n", i == -1 ? "" : no_hash_pointers_warning[lines[i]]);
> >
> > Is this worth it, please? Could anyone provide some numbers how
>
> Yeah, the code indeed starts to look a bit cumbersome...
>
> > the kernel size increases between releases?
>
> I'd say 20 KiB per release, on average.
>
> > The number of code lines is basically just growing. The same is true
> > for the amount of printed messages.
>
> Yeah, we keep on adding more messages.
> But do we really need to print a message of 13 lines?
> If you consider this critical for security, perhaps it should use pr_crit(),
> or pr_alert()? But please don't print more than a single line.
>
> <sarcastic>
> Perhaps it should print a URL to a message instead, like the
> "software license" option in Android systems and apps?
> </sarcastic>
>
> > This patch is saving some lines of text that might be effectively
> > compressed. But it adds some code and array with indexes. Does it
> > make any significant imrovement in the compressed kernel image?
> >
> > Geert was primary concerned about the runtime memory consuption.
> > It will be solved by the __initconst. The rest affects only
> > the size of the compressed image on disk.
>
> I'm actually concerned about both. Platforms (and boot loaders) may
> have limitations for kernel image size, too.
> Static memory consumption is also more easily measured, so I tend
> to run bloat-o-meter, and dive into anything that adds more than 1 KiB.
> And yes, this message is a low-hanging fruit...

OK, I wondered how big trick does the __initconst on its own.

1. I compiled kernel without this patchset:

$# ll /boot/vmlinux-5.12.0-rc2-default+.bz2
-rwxr-xr-x 1 root root 18911364 Mar 8 15:58 /boot/vmlinux-5.12.0-rc2-default+.bz2

2. With this patchset:

$# ll /boot/vmlinux-5.12.0-rc2-default+.bz2
-rwxr-xr-x 1 root root 18910767 Mar 8 16:16 /boot/vmlinux-5.12.0-rc2-default+.bz2
$# echo $((18910767 - 18911364))
-597

3. With the patch below:

$# ll /boot/vmlinux-5.12.0-rc2-default+.bz2
-rwxr-xr-x 1 root root 18910906 Mar 8 16:58 /boot/vmlinux-5.12.0-rc2-default+.bz2
$# echo $((18910906 - 18911364))
-458

This patchset saves 139B more than a simple array.

Well, I am a bit confused. I have tried to keep the strings as a
static variable outside the function:

static const char *no_hash_pointers_warning[] __initconst = {
...

and I got the following build error:

CC lib/vsprintf.o
lib/vsprintf.c:2097:20: error: no_hash_pointers_warning causes a section type conflict with __setup_str_no_hash_pointers_enable
static const char *no_hash_pointers_warning[] __initconst = {
^~~~~~~~~~~~~~~~~~~~~~~~
In file included from ./include/linux/printk.h:6:0,
from ./include/linux/kernel.h:16,
from ./include/linux/clk.h:13,
from lib/vsprintf.c:22:
./include/linux/init.h:315:20: note: ‘__setup_str_no_hash_pointers_enable’ was declared here
static const char __setup_str_##unique_id[] __initconst \
^
./include/linux/init.h:330:2: note: in expansion of macro ‘__setup_param’
__setup_param(str, fn, fn, 1)
^~~~~~~~~~~~~
lib/vsprintf.c:2127:1: note: in expansion of macro ‘early_param’
early_param("no_hash_pointers", no_hash_pointers_enable);
^~~~~~~~~~~

I solved this be defining the array inside the function that is marked
__init. But I am not sure if it is the correct solution. And I wonder
why the original patch did not have this problem.

Also I am curious why the array reduced the size of the binary so
significantly in compare with the const strings used as pr_warn()
arguments. It might depend on the compression method or???

Anyway, here is the patch that works for me and reduced the size of
the binary considerably:

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 4a14889ccb35..af01edae0d86 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2096,24 +2096,30 @@ EXPORT_SYMBOL_GPL(no_hash_pointers);

static int __init no_hash_pointers_enable(char *str)
{
+ int i;
+ const char *no_hash_pointers_warning[] = {
+ "**********************************************************",
+ "** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **",
+ "** **",
+ "** This system shows unhashed kernel memory addresses **",
+ "** via the console, logs, and other interfaces. This **",
+ "** might reduce the security of your system. **",
+ "** **",
+ "** If you see this message and you are not debugging **",
+ "** the kernel, report this immediately to your system **",
+ "** administrator! **",
+ "** **",
+ "** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **",
+ "**********************************************************",
+ };
+
if (no_hash_pointers)
return 0;

no_hash_pointers = true;

- pr_warn("**********************************************************\n");
- pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
- pr_warn("** **\n");
- pr_warn("** This system shows unhashed kernel memory addresses **\n");
- pr_warn("** via the console, logs, and other interfaces. This **\n");
- pr_warn("** might reduce the security of your system. **\n");
- pr_warn("** **\n");
- pr_warn("** If you see this message and you are not debugging **\n");
- pr_warn("** the kernel, report this immediately to your system **\n");
- pr_warn("** administrator! **\n");
- pr_warn("** **\n");
- pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
- pr_warn("**********************************************************\n");
+ for (i = 0; i < ARRAY_SIZE(no_hash_pointers_warning); i++)
+ pr_warn("%s\n", no_hash_pointers_warning[i]);

return 0;
}

Honestly, I do not want to spend much more time on this. I made the
test out of curiosity.

Feel free to provide the patch using the array, ideally with some
numbers how it helps. But please _avoid_ the indirection via

const int lines[] = { 0, 1, -1, 2, 3, 4, -1, 5, 6, 7, -1, 1, 0 };

and also _avoid_ all the hardcoded constants, like:

no_hash_pointers_warning[8][55]

and

pr_warn("**%54s**\n"

They are error prone and hard to maintain. Such tricks are not
worth it from my POV.

Best Regards,
Petr