Hi Pratyush,

Thanks for your reply. Please see my replies inline:

On Wed, May 30, 2018 at 8:05 AM, Pratyush Anand
[off-list ref] wrote:

Hi Bhupesh,


On Mon, May 28, 2018 at 2:33 AM, Bhupesh Sharma [off-list ref] wrote:

quoted

Hi ARM64 maintainers,

[...]

quoted

4. Since 'memstart_addr' indicates the start of physical RAM, we
randomize the same on basis of 'memstart_offset_seed' value above.
Also the 'memstart_addr' value is available in '/proc/kallsyms' and
hence can be accessed by user-space applications to read the
'memstart_addr' value.

User space can read, only when a system has either CONFIG_DEVMEM or
CONFIG_PROC_KCORE  enabled. But, we can have a system which has
CONFIG_RANDOMIZE_BASE enabled but none of the kernel memory access
interface (other than kdump/vmcore).

quoted

5. Now since the PAGE_OFFSET value is also used by several user space
tools (for e.g. makedumpfile tool uses the same to determine the start
of linear region and hence to read PT_NOTE fields from /proc/kcore), I
am not sure how to read the randomized value of the same in the KASLR
enabled case.

Do we have a use case other than makedumpfile (I mean where we do not
have vmcore access)? For makedumpfile, we have following information
in vmcore, and I hope, that should be sufficient to find physical
address of any symbol from vmcore.

358         VMCOREINFO_NUMBER(VA_BITS);
359         /* Please note VMCOREINFO_NUMBER() uses "%d", not "%x" */
360         vmcoreinfo_append_str("NUMBER(kimage_voffset)=0x%llx\n",
361                                                 kimage_voffset);
362         vmcoreinfo_append_str("NUMBER(PHYS_OFFSET)=0x%llx\n",
363                                                 PHYS_OFFSET);

Please see more details in a mail I sent as a reply to Ard (in this
thread itself), but one specific use case that I am working on at the
moment is the makedumpfile '--mem-usage' for KASLR enabled arm64
kernels.

This use case requires directly reading the '/proc/kcore' and the
hence the PAGE_OFFSET value is used to determine the base address of
the linear region, whose value is not static in case of KASLR boot.

Another use-case is where the crash-utility uses the PAGE_OFFSET value
to perform a virtual-to-physical conversion for the address lying in
the linear region:

ulong
arm64_VTOP(ulong addr)
{
    if (machdep->flags & NEW_VMEMMAP) {
        if (addr >= machdep->machspec->page_offset)
            return machdep->machspec->phys_offset
                + (addr - machdep->machspec->page_offset);

<..snip..>
}

There might be other use-cases (which I am unaware about), but
presently I am seeing issues with the above two use-cases in case of
KASLR enabled arm64 kernels and trying to find out possible acceptable
solutions.

Regards,
Bhupesh