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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