Re: [PATCH v4 3/4] tools/bootconfig: Align the bootconfig applied initrd image size to 4

From: Masami Hiramatsu
Date: Wed Nov 18 2020 - 21:32:48 EST


On Wed, 18 Nov 2020 11:22:49 -0500
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> On Thu, 19 Nov 2020 00:35:44 +0900
> Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> > +
> > + /* To align up the total size to BOOTCONFIG_ALIGN, get padding size */
> > + total_size = stat.st_size + size + sizeof(u32) * 2 + BOOTCONFIG_MAGIC_LEN;
> > + pad = BOOTCONFIG_ALIGN - total_size % BOOTCONFIG_ALIGN;
> > + if (pad == BOOTCONFIG_ALIGN)
> > + pad = 0;
>
> If alignment is always a power of two, you could simply do:
>
> pad = (total_size + BOOTCONFIG_ALIGN - 1) & ~(BOOTCONFIG_ALIGN - 1)) - total_size;
>
> Which will give you the proper padding, without the if.

Thanks, and in that case, I would like to introduce BOOTCONFIG_ALINE_BITS and
BOOTCONFIG_ALINE_MASK so that we make sure the alignment is power of two.

> > + size += pad;
> > +
> > + /* Add a footer */
> > + *(u32 *)(data + size) = size;
> > + *(u32 *)(data + size + sizeof(u32)) = csum;
> > + memcpy(data + size + sizeof(u32) * 2, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN);
> > + total_size = size + sizeof(u32) * 2 + BOOTCONFIG_MAGIC_LEN;
>
> I wonder if it would be cleaner to just have a void pointer index for the above:
>
> void *p;
>
> p = data + size;
> *(u32 *)p = size;
> p += sizeof(u32);
>
> *(u32 *)p = csum;
> p += sizeof(u32);
>
> memcpy(p, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN);
> p += BOOTCONFIG_MAGIC_LEN;
>
> total_size = p - (void *)data;

OK.

> Also, how does this work if we run this on a little endian box for a big
> endian crossbuild?

Hmm, good point. I expected that the bootconfig command will be used
natively. Hmm, there are 2 options.

- Add endian option to the bootconfig (-le/-be) for cross build.
- Update the footer format to use "ascii" size and checksum.

To generalize the format, latter one is better. But it also involves
the kernel side update too.

Thank you,

>
> -- Steve
>
>
> > +
> > + ret = write(fd, data, total_size);
> > + if (ret < total_size) {
> > if (ret < 0)
> > ret = -errno;
> > pr_err("Failed to apply a boot config: %d\n", ret);
> > - if (ret < 0)
> > - goto out;
> > - goto out_rollback;
> > - }
> > - /* Write a magic word of the bootconfig */
> > - ret = write(fd, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN);
> > - if (ret < BOOTCONFIG_MAGIC_LEN) {
> > - if (ret < 0)
> > - ret = -errno;
> > - pr_err("Failed to apply a boot config magic: %d\n", ret);
> > - goto out_rollback;
> > - }
> > - ret = 0;
> > + if (ret > 0)
> > + goto out_rollback;
> > + } else
> > + ret = 0;
> > +
> > out:
> > close(fd);
> > free(data);


--
Masami Hiramatsu <mhiramat@xxxxxxxxxx>