Re: [Crash-utility] "cannot access vmalloc'd module memory" when loading kdump'ed vmcore in crash
----- "Dave Anderson" <anderson(a)redhat.com> wrote:
----- "Kevin Worth" <kevin.worth(a)hp.com> wrote:
> Thanks, Dave. Is it valid to just do "make modules" since it
appears
> we're just adding a module or does the modification to
> arch/i386/mm/init.c necessitate a rebuilt kernel?
You might be able to *build* crash.o with "make modules", but if
you try to install it, it's going to fail due because it won't
be able to resolve the "page_is_ram" reference.
There may be some other way to export a symbol from
the base kernel without rebuilding the kernel. I have
seen some 3rd-party modules (i.e., non-Red Hat) that
load a "rogue" module that tinkers with its own internal
exported symbol list after it is installed by overwriting
its own exported symbols with the symbol name and address
of un-exported base kernel symbols. Then, after the rogue module
gets installed (and overwrites its own list of exported symbols),
a second "real" module gets installed -- and the real module
uses the illegally-exported (?) kernel symbols from the first
rogue module. Seems like a violation of the GPL, but anyway,
I don't have any examples of how they do it.
Another thing (potentially dangerous) you could try to avoid
a kernel rebuild would be to just remove the page_is_ram() call
from crash.h:
map_virtual(u64 offset, struct page **pp)
{
struct page *page;
unsigned long pfn;
void *vaddr;
pfn = (unsigned long)(offset >> PAGE_SHIFT);
if (!page_is_ram(pfn)) {
printk(KERN_INFO
"crash memory driver: !page_is_ram(pfn: %lx)\n", pfn);
return NULL;
}
if (!pfn_valid(pfn)) {
printk(KERN_INFO
"crash memory driver: invalid pfn: %lx )\n", pfn);
return NULL;
}
page = pfn_to_page(pfn);
vaddr = kmap(page);
if (!vaddr) {
printk(KERN_INFO
"crash memory driver: pfn: %lx kmap(page: %lx) failed\n",
pfn, (unsigned long)page);
return NULL;
}
*pp = page;
return (vaddr + (offset & (PAGE_SIZE-1)));
}
Then you just hope that pfn_valid() will be safe enough.
Dave