Messages in this thread Patch in this message | | | Date | Tue, 8 Aug 2006 15:36:34 -0400 | From | Don Zickus <> | Subject | Re: [Fastboot] [CFT] ELF Relocatable x86 and x86_64 bzImages |
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On Mon, Aug 07, 2006 at 11:01:53PM -0600, Eric W. Biederman wrote: > Don Zickus <dzickus@redhat.com> writes: > > >> >> > > >> >> >> > >> >> >> The error is the uncompressed length does not math the stored length > >> >> >> of the data before from before we compressed it. Now what is > >> >> >> fascinating is that our crc's match (as that check is performed first). > >> >> >> > >> >> >> Something is very slightly off and I don't see what it is. > >> >> > > >> >> > I printed out orig_len -> 5910532 (which matches vmlinux.bin) > >> >> > bytes_out -> 5910531 > >> >> > > >> >> >> > >> > It seems to be an AMD64 vs EM64T problem. AMD chipsets work but Intel > >> > chipsets don't. > >> > > >> > I also blindly incremented bytes_out (as a really cheap hack), it didn't > >> > work until I added some random putstr's below it (timing??). Then the > >> > kernel booted. > >> > > >> > Still looking into things. > >> > >> Odd. I wonder if I'm missing a serializing instruction somewhere, > >> to ensure the effects of ``self modifying code'' aren't a problem. > >> As I read Intels Documentation if you have a jump before you get > >> to the code there shouldn't be a problem. > >> > >> Still that doesn't really explain bytes_out. > >> > > > > So I narrowed down the problem but it isn't obvious to me why this problem > > exists. Basically, even though bytes_out is supposed to be initialized to > > 0, it becomes -1 before entering decompress_kernel(). Of course, the > > fallout is in flush_window() bytes_out wounds up being one less than > > outcnt and hence my original problem. > > > > Any thoughts on how to debug where this could be getting corrupted? > > Looking at my build it appears bytes_out is being placed in the .bss. > A little odd since it is zero initialized but no big deal. > Could you confirm that bytes_out is being placed in the .bss section > by inspecting arch/x86_64/boot/compresssed/misc.o and > arch/x86_64/boot_compressed/vmlinux. "readelf -a $file" and then > looking up the section number and looking at the section table to see > which section it is was my technique.
Yes bytes_out is in the .bss for both files.
> > If bytes_out is in the .bss for you then I suspect something is not > correctly zeroing the .bss. Or else the .bss is being stomped. > > I'm not certain how rep stosb can be done wrong but some bad pointer > math could have done it.
Even worse, from the time the .bss is cleared to the time gunzip() is called inside decompress_kernel(), there is very little code to do some stomping.
So I am stuck trying to debug this. This code seems very fragile. The more debug code I add (ie putstr) the more the length is off (varies from -32 to +1). Makes me scratch my head as to what is really going on here.
I created a really pathetic patch to get the thing to boot but even that doesn't make sense.
diff --git a/arch/x86_64/boot/compressed/misc.c b/arch/x86_64/boot/compressed/misc.c index 0e6c4b7..614416e 100644 --- a/arch/x86_64/boot/compressed/misc.c +++ b/arch/x86_64/boot/compressed/misc.c @@ -183,6 +183,7 @@ #define OLD_CL_MAGIC 0xA33F extern unsigned char input_data[]; extern int input_len; +static long dummy; static long bytes_out = 0; static void *malloc(int size); @@ -594,6 +595,7 @@ asmlinkage void decompress_kernel(void * if ((ulg)output >= 0xffffffffffUL) error("Destination address too large"); + bytes_out = 0; makecrc(); putstr(".\nDecompressing Linux..."); gunzip(); And yes, the 'dummy' variable needs to be there. I am trying to use gdb on vmlinux to fish for clues. But I am at a loss right now.
Cheers, Don
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