[Crash-utility] [PATCH] Expand search for banner string to get phys_base

[江冉]

Hi Dave,

When tring to debug a kernel core file generated from Debian 9.2 + 4.9 kernel with base address randomized (this core is create for a qemu vm), it gives the error as:

WARNING: cannot determine physical base address: defaulting to 0

GNU gdb (GDB) 7.6
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This GDB was configured as "x86_64-unknown-linux-gnu"...

WARNING: failed to init kexec backup region
WARNING: cannot read linux_banner string

Taking a look at the code around x86_64_virt_phys_base(), it determines phys_base by searching around machdep->machspec->phys_base(in my case it's 0) +/-16MB for banner string at banner string offset (get from symbol). However, with base address randomized, the real physical address could be far from 0 (or virtual address 0xffffffff80000000). Therefore, -16MB/+16MB is not enough especially for machine with large ram . I increased it to 0xFFFFF00000 for my 256g core file and it works.

-        for (phys = (ulong)(-MEGABYTES(16)); phys != MEGABYTES(16+1);
+       for (phys = (ulong)(-MEGABYTES(32)); phys != 0xFFFFF00000;
             phys += MEGABYTES(1)) {
                if (readmem(linux_banner_phys + phys, PHYSADDR, buf,
                    strlen("Linux version"), "linux_banner search",
                    QUIET|RETURN_ON_ERROR) && STRNEQ(buf, "Linux version")) {
                        if (CRASHDEBUG(1))
                                fprintf(fp,
                                    "virtual dump phys_base: %lx %s\n", phys,
                                        machdep->machspec->phys_base != phys ?
                                        "override" : "");
                        machdep->machspec->phys_base = phys;
                        return TRUE;
                }
        }

Please feel free to let me know if you need sample dumps to verify. Thx.

Regards,
Ran
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