Re: [PATCH V2] proc-vmcore: wrong data type casting fix

[Baoquan He <bhe@redhat.com>]

On 03/14/16 at 11:47am, Baoquan He wrote:
> On 03/14/16 at 12:25pm, HATAYAMA Daisuke wrote:
> > From: Dave Young <dyoung@redhat.com>
> > Subject: [PATCH V2] proc-vmcore: wrong data type casting fix
> > Date: Fri, 11 Mar 2016 16:42:48 +0800
> > 
> > > On i686 PAE enabled machine the contiguous physical area could be large
> > > and it can cause trimming down variables in below calculation in
> > > read_vmcore() and mmap_vmcore():
> > > 
> > > 	tsz = min_t(size_t, m->offset + m->size - *fpos, buflen);
> > > 
> > > Then the real size passed down is not correct any more.
> > > Suppose m->offset + m->size - *fpos being truncated to 0, buflen >0 then
> > 
> > That is, size_t and loff_t are defined as follows on i686:
> > 
> >     (gdb) ptype size_t
> >     type = unsigned int
> >     (gdb) ptype loff_t
> >     type = long long int
> > 
> > So casting by size_t means truncating a given value by 4GB.
> > 
> > Then, if (m->offset + m->size - *fpos) is equal to or larger than 4GB,
> > and is aligned with 4GB, the resulted value is 0, and
> 
> Truncating doesn't mean align. If (m->offset + m->size - *fpos) is
> larger than 4G, E.g 0x10000000f, the truncating result will be 0xf.
> 
> > 
> > > we will get tsz = 0. It is of course not an expected result.
> 
> We won't always get "tsz=0", just get the lower 32 bit value.

OK, didn't get you still have saying in next paragraph. Ignore this
please.

> 
> > > 
> > > During our tests there are two problems caused by it:
> > > 1) read_vmcore will refuse to continue so makedumpfile fails.
> > > 2) mmap_vmcore will trigger BUG_ON() in remap_pfn_range().
> > >
> > 
> > we reach these errors.
> > 
> > If (m->offset + m->size - *fpos) is not aligned with 4GB,
> > read_vmcore() or mmap_vmcore() is performed with the truncated
> > non-zero value as size (of course, this is also not expected value but
> > the execution doesn't result in error). Then, fpos proceeds so that
> > (m->offset + m->size - *fpos) is aligned with 4GB in the next loop,
> > and we after all reach the errors.
> > 
> > I think your patch description needs a bit more detail.
> > 
> > It seems good to me that the patch itself.
> > 
> > > Use unsigned long long in min_t instead so that the variables are not
> > > truncated.
> > > 
> > > Signed-off-by: Baoquan He <bhe@redhat.com>
> > > Signed-off-by: Dave Young <dyoung@redhat.com>
> > > ---
> > > v1->v2: spelling fix in patch log
> > >  fs/proc/vmcore.c |    7 +++++--
> > >  1 file changed, 5 insertions(+), 2 deletions(-)
> > > 
> > > --- linux-x86.orig/fs/proc/vmcore.c
> > > +++ linux-x86/fs/proc/vmcore.c
> > > @@ -231,7 +231,9 @@ static ssize_t __read_vmcore(char *buffe
> > >  
> > >  	list_for_each_entry(m, &vmcore_list, list) {
> > >  		if (*fpos < m->offset + m->size) {
> > > -			tsz = min_t(size_t, m->offset + m->size - *fpos, buflen);
> > > +			tsz = (size_t)min_t(unsigned long long,
> > > +					    m->offset + m->size - *fpos,
> > > +					    buflen);
> > >  			start = m->paddr + *fpos - m->offset;
> > >  			tmp = read_from_oldmem(buffer, tsz, &start, userbuf);
> > >  			if (tmp < 0)
> > > @@ -461,7 +463,8 @@ static int mmap_vmcore(struct file *file
> > >  		if (start < m->offset + m->size) {
> > >  			u64 paddr = 0;
> > >  
> > > -			tsz = min_t(size_t, m->offset + m->size - start, size);
> > > +			tsz = (size_t)min_t(unsigned long long,
> > > +					    m->offset + m->size - start, size);
> > >  			paddr = m->paddr + start - m->offset;
> > >  			if (vmcore_remap_oldmem_pfn(vma, vma->vm_start + len,
> > >  						    paddr >> PAGE_SHIFT, tsz,
> > > 
> > --
> > Thanks.
> > HATAYAMA, Daisuke

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