[U-Boot] Issue with memcpy when booting a fitImage on SPEAr600

[Thomas Petazzoni]

Hello Stefan,

On Wed, 5 Dec 2018 12:30:57 +0100, Stefan Roese wrote:

> > It's weird to me that it's happening with this SoC because it's based on
> > ARM926ejs which is widely used I assume. Shouldn't have anyone already
> > encountered the bug? Or is nobody actually booting a fitImage and had the
> > luck to never call memcpy with src == dest anywhere else in the code?  
> 
> I did some work on the SPEAr600 some years ago and I'm pretty sure that
> I never used FIT image at that time. Sorry, but I can't remember any
> similar issues like these.

Well, the issue is in generic ARM code, so whether it's SPEAr600 or any
other ARM SoC should not really matter here.

> FWIW, I would not oppose to having at least this "src == dest" check
> in the code again, since it doesn't really make sense to overwrite
> an area with the same value - other than for testing purposes.

The thing is that the memcpy() that gets called does have this src ==
dest check, as its code starts with:

ENTRY(memcpy)
                cmp     r0, r1
                bxeq    lr

which, if my assembly-fu is not bad, means: if first argument == second
argument, then return. But even with this src == dest check within
memcpy() itself, Quentin is seeing memmove() hang when src == dest.

Another thing is that the memmove() code looks like this:

{
        char *tmp, *s;

        if (dest <= src) {
                memcpy(dest, src, count);

However, having dest <= src does not guarantee that the destination and
source areas are non-overlapping. And the normal semantic for memcpy()
is that it doesn't work for overlapping areas. From memcpy(3):

  The memcpy() function copies n bytes from memory area src to memory
  area dest.  The memory areas must not overlap.  Use memmove(3) if the
  memory areas do overlap.

Of course, this man page documents the memcpy() implementation from the
C library, and perhaps the semantic of U-Boot memcpy is different.

So is it correct to use memcpy() to implement memmove() when the areas
are overlapping ?

Perhaps Simon Glass, who did the change to use memcpy() in
cb0eae8cf8aaca76910dee4c7eb536d0814d1bd2 could comment on that ?

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com