[makedumpfile PATCH RFC v0.1] Implemented the --fill-excluded-pages=<value> feature

[Eric DeVolder]

On 07/20/2017 08:20 AM, Dave Anderson wrote:
> 
> 
> ----- Original Message -----
> 
>> When a page is excluded by any of the existing dump levels,
>> that page may still be written to the ELF dump file, depending
>> upon the PFN_EXCLUDED mechanism.
>>
>> The PFN_EXCLUDED mechanism looks for N consecutive "not
>> dumpable" pages, and if found, the current ELF segment is
>> closed out and a new ELF segment started, at the next dumpable
>> page. Otherwise, if the PFN_EXCLUDED criteria is not meet (that
>> is, there is a mix of dumpable and not dumpable pages, but not
>> N consecutive not dumpable pages) all pages are written to the
>> dump file.
>>
>> This patch implements a mechanism for those "not dumpable" pages
>> that are written to the ELF dump file to fill those pages with
>> constant data, rather than the original data. In other words,
>> the dump file still contains the page, but its data is wiped.
>>
>> The motivation for doing this is to protect real user data from
>> "leaking" through to a dump file when that data was asked to be
>> omitted. This is especially important for effort I currently am
>> working on to allow further refinement of what is allowed to be
>> dumped, all in an effort to protect user (customer) data.
>>
>> The patch is simple enough, however, it causes problems with
>> crash; crash is unable to load the resulting ELF dump file.
>> For example, I do the following as a test scenario for this
>> change:
>>
>> - Obtain a non-filtered dump file (eg. dump level 0, no -d option,
>>    or straight copy of /proc/vmcore)
>> - Run vmcore through 'crash' to ensure loads ok, test with
>>    commands like: ps, files, etc.
>>    % crash vmlinux vmcore
>> - Apply this patch and rebuild makedumpfile
>> - Run vmcore through makedumpfile *without* --fill-excluded-pages
>>    and with filtering to ensure no uintended side effects of patch:
>>    % ./makedumpfile -E -d31 -x vmlinux vmcore newvmcore
>> - Run new vmcore through crash to ensure still loads ok, test
>>    with commands like: ps, files, etc.
>>    % crash vmlinux newvmcore
>> - Run vmcore through makedumpfile *with* --fill-excluded-pages
>>    and with filtering to check side effects of patch:
>>    % ./makedumpfile -E -d31 --fill-excluded-pages=0 -x vmlinux vmcore
>>    newvmcore2
>> - Run new vmcore through crash to ensure still loads ok, test
>>    with commands like: ps, files, etc.
>>    % crash vmlinux newvmcore2
>>
>> But crash yields errors like:
>>    [...]
>>    This GDB was configured as "x86_64-unknown-linux-gnu"...
>>
>>    crash: cannot determine thread return address
>>    please wait... (gathering kmem slab cache data)
>>    crash: invalid kernel virtual address: 1c  type: "kmem_cache
>>    objsize/object_size"
>>
>> If the patch is correct/accurate, then that may mean that crash
>> is using data which it should not be.
> 
> Why would the crash utility be "using data which it should not be"
> if your patch is applied?
> 
> The two error messages above come from attempting to read memory
> from the kernel text region (the "thread return" message), and then
> the kmem_cache.object_size field of the kernel's kmem_cache data structure
> pointed to by its "kmem_cache" pointer.  It looks like the patch is
> causing bogus data to be returned for a given physical address >
> Dave
> 

Indeed, the patch was incorrect and was causing bogus data to be 
returned. I've corrected the patch and will re-post soon.

Eric
> 
>>
>> The more likely scenario is that the patch is not correct/accurate,
>> and I'm corrupting the dump file.
>>
>> Please provide feedback!!
> 
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