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

[HATAYAMA Daisuke]

From: Baoquan He <bhe@xxxxxxxxxx>
Subject: Re: [PATCH V2] proc-vmcore: wrong data type casting fix
Date: Mon, 14 Mar 2016 11:50:53 +0800

> On 03/14/16 at 11:47am, Baoquan He wrote:
>> On 03/14/16 at 12:25pm, HATAYAMA Daisuke wrote:
>> > From: Dave Young <dyoung@xxxxxxxxxx>
>> > 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.
>>

Thanks for pointing out. I was confused with truncation and
alignment. I wanted to say here...

>> > 
>> > > 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.

truncating the upper 32 bits and just getting the lower 32 bits as you
say.

> 
> 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@xxxxxxxxxx>
>> > > Signed-off-by: Dave Young <dyoung@xxxxxxxxxx>
>> > > ---
>> > > 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
> 
--
Thanks.
HATAYAMA, Daisuke