Re: bootconfig length parse error in kernel

[Chen Yu <yu.chen.surf@gmail.com>]

Hi Masami,

On Wed, Nov 11, 2020 at 5:37 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> Hi Chen,
>
> On Tue, 10 Nov 2020 23:39:53 +0800
> Chen Yu <yu.chen.surf@gmail.com> wrote:
>
> > Hi Masami,
> > Thanks for writing bootconfig and it is useful for boot up trace event
> > debugging.
>
> Thanks for testing!
>
> > However it was found that on 5.10-rc2 the bootconfig does not work and it shows
> > "'bootconfig' found on command line, but no bootconfig found"
> > And the reason for this is the kernel found the magic number to be incorrect.
> > I've added some hack in kernel to dump the first 12 bytes, it shows:
> > "OTCONFIG". So printed more content ahead we can find
> > "#BOOTCONFIG" ahead. So it looks that there is some alignment during
> > initrd load, and get_boot_config_from_initrd() might also deal with it. That is
> > to say:
> > data = (char *)initrd_end - BOOTCONFIG_MAGIC_LEN;
> > might do some alignment?
>
> Hrm, interesting. So initrd_end might be aligned. Could you print out the
> actuall address of initrd_end?
I've done some investigation, it looks like this issue is not related
to alignment, but related to
the bootloader that has provided an inaccurate ramdisk size via
boot_params.hdr.ramdisk_size.
The actual size of initrd is:
ls /boot/initrd.img-5.10.0-rc3-e1000e-hw+ -l
-rw-r--r-- 1 root root 48689230 11月 12 00:08
/boot/initrd.img-5.10.0-rc3-e1000e-hw+
while the ramdisk size provided by bootloader via
boot_params.hdr.ramdisk_size is
48689232, which is 2 bytes bigger than the actual size, and this is
why the initrd_end
is bigger than expected and causing the missmatch of magic number.
Since there is no guarantee that bootloader provides the accurate
ramdisk size, an compromised
proposal might be that to search for the magic number a little ahead.
For example, the
following patch works for me:
diff --git a/init/main.c b/init/main.c
index 130376ec10ba..60fb125d44f4 100644
--- a/init/main.c
+++ b/init/main.c
@@ -273,7 +273,10 @@ static void * __init
get_boot_config_from_initrd(u32 *_size, u32 *_csum)
        if (!initrd_end)
                return NULL;

-       data = (char *)initrd_end - BOOTCONFIG_MAGIC_LEN;
+       data = memchr((char *)initrd_end - 2 * BOOTCONFIG_MAGIC_LEN,
+                      '#', BOOTCONFIG_MAGIC_LEN);
+       if (!data)
+               return NULL;
        if (memcmp(data, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN))
                return NULL;


> And could you tell me which platform are you tested?
>
It is HP ZHAN 99 Mobile Workstation G1 with i5-8300H, Ubuntu 20.04.

Thanks,
Chenyu

> Thank you,
>
> --
> Masami Hiramatsu <mhiramat@kernel.org>