Re: [PATCH 3/5] arm: decompressor: define a new zImage tag

On Mon, Jun 01, 2020 at 10:27:45PM +0200, Lukasz Stelmach wrote:

It was <2020-06-01 pon 19:25>, when Russell King - ARM Linux admin wrote:

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On Mon, Jun 01, 2020 at 06:19:52PM +0200, Lukasz Stelmach wrote:

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It was <2020-06-01 pon 15:55>, when Russell King - ARM Linux admin wrote:

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On Mon, Jun 01, 2020 at 04:27:52PM +0200, Łukasz Stelmach wrote:

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Add DCSZ tag which holds dynamic memory (stack, bss, malloc pool)
requirements of the decompressor code.

Why do we need to know the stack and BSS size, when the userspace
kexec tool doesn't need to know this to perform the same function.

kexec-tools load zImage as low in DRAM as possible and rely on two
assumptions:

    + the zImage will copy itself to make enough room for the kernel,
    + sizeof(zImage+mem) < sizeof(kernel+mem), which is true because
      of compression.

       DRAM start
       + 0x8000

zImage    |-----------|-----|-------|
            text+data   bss   stack 

                 text+data           bss   
kernel    |---------------------|-------------------|


initrd                                              |-initrd-|-dtb-|

This is actually incorrect, because the decompressor will self-
relocate itself to avoid the area that it is going to decompress
into.

I described the state right after kexec(8) invocation.

Actually, you haven't, because this is _not_ how kexec(8) lays it
out, as I attempted to detail further down in my reply.

quoted

So, while the decompressor runs, in the above situation it
ends up as:


ram    |------------------------------------------------------...
                 text+data           bss   
kernel    |---------------------|-------------------|
zImage                          |-----------|-----|-------|
                                  text+data   bss   stack+malloc

Note here - if the initrd was placed as _you_ describe at the end
of where the zImage ends up including its BSS, it would be
corrupted by the stack and malloc space of the decompressor while
running.  Ergo, your description of how kexec(8) lays stuff out
is incorrect.

quoted

Where "text+data" is actually smaller than the image size that
was loaded - the part of the image that performs the relocation
is discarded (the first chunk of code up to "restart" - 200
bytes.)  The BSS is typically smaller than 200 bytes, so we've
been able to get away without knowing the actual BSS size so
far.


ram    |--|-----------------------------------------|---------...
       |<>| TEXT_OFFSET
kernel    |---------------------|-------------------|
          |<----edata_size----->|<-----bss_size---->|
          |<---------------kernel_size------------->|
zImage                          |-----------|-----|-------|
                                |<-------len------->| (initial)
				|<----------len------------>| (final)

The "final" len value is what the decompressor prints as the "zImage
requires" debugging value.

Hence, the size that the decompressed kernel requires is kernel_size.

The size that the decompressor requires is edata_size + len(final).

Now, if you intend to load the kernel to ram + TEXT_OFFSET + edata_size
then it isn't going to lose the first 200 bytes of code, so as you
correctly point out, we need to know the BSS size.

Formal note: can we keep using terms zImage and kernel as separate,
where zImage is what is loaded with kexec and kernel is the decompressed
code loaded at TEXT_OFFSET. I believe, it will help us avoid mistakes.

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+struct arm_zimage_tag_dc {
+	struct tag_header hdr;
+	union {
+#define ZIMAGE_TAG_DECOMP_SIZE ARM_ZIMAGE_MAGIC4
+		struct zimage_decomp_size {
+			__le32 bss_size;
+			__le32 stack_size;
+			__le32 malloc_size;
+		} decomp_size;

You certainly don't need to know all this.  All you need to know is
how much space the decompressor requires after the end of the image,
encompassing the BSS size, stack size and malloc size, which is one
value.

I agree. However, since we are not fighting here for every single byte,
I'd rather add them as separate values and make the tag, even if only
slightly, more future-proof.

It doesn't make it more future-proof.  What happens if we add something
else, do we need to list it separately, and then change the kernel to
accept the new value and maybe also kexec(8)?  Or do we just say "the
decompressor needs X many bytes after the image" and be done with it?
The latter sounds way more future-proof to me.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC for 0.8m (est. 1762m) line in suburbia: sync at 13.1Mbps down 424kbps up

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