Since commit 23c85094fe1895caefdd ["proc/kcore: add vmcoreinfo note to /proc/kcore"]), '/proc/kcore' contains a new PT_NOTE which carries the VMCOREINFO information.
If the same is available, one can use it in user-land to retrieve machine specific symbols or strings being appended to the vmcoreinfo even for live-debugging of the primary kernel as a standard interface exposed by kernel for sharing machine specific details with the user-land. In the past I had a discussion with James, where he suggested this approach (please see [0]) and I really liked the idea. Since then I have been working on unifying the implementations of (atleast) the commonly used user-space utilities that provide live-debugging capabilities (tools like 'makedumpfile' and 'crash-utility', see [1] for details of these tools). For the same, when live debugging on x86_64 machines, user-space tools currently rely on different mechanisms to determine the 'page_offset_base' value (i.e. start of direct mapping of all physical memory). One of the approach used by 'makedumpfile' user-space tool for e.g. is to calculate the same from the last PT_LOAD available in '/proc/kcore', which can be flaky as and when new sections (for e.g. KCORE_REMAP which was added to recent kernels) are added to kcore. For other architectures like arm64, I have already proposed using the vmcoreinfo note (in '/proc/kcore') in the user-space utilities to determine machine specific details like VA_BITS, PAGE_OFFSET, kasrl_offset() (see [2] for details), for which different user-space tools earlier used different (and at times flaky) approaches like: - Reading kernel CONFIGs from user-space and determining CONFIG values like VA_BITS from there. - Reading symbols from '/proc/kallsyms' and determining their values via '/dev/mem' interface. - Reading symbols from 'vmlinux' and determing their values from reading memory. This patch allows appending 'page_offset_base' for x86_64 platforms to vmcoreinfo, so that user-space tools can use the same as a standard interface to determine the start of direct mapping of all physical memory. Testing: ------- - I tested this patch (rebased on 'linux-next') on a x86_64 machine using the modified 'makedumpfile' user-space code (see [3] for my github tree which contains the same) for determining how many pages are dumpable when different dump_level is specified (which is one use-case of live-debugging via 'makedumpfile'). - I tested both the KASLR and non-KASLR boot cases with this patch. - Here is one sample log (for KASLR boot case) on my x86_64 machine: < snip..> The kernel doesn't support mmap(),read() will be used instead. TYPE PAGES EXCLUDABLE DESCRIPTION ---------------------------------------------------------------------- ZERO 21299 yes Pages filled with zero NON_PRI_CACHE 91785 yes Cache pages without private flag PRI_CACHE 1 yes Cache pages with private flag USER 14057 yes User process pages FREE 740346 yes Free pages KERN_DATA 58152 no Dumpable kernel data page size: 4096 Total pages on system: 925640 Total size on system: 3791421440 Byte I understand that there might be some reservations about exporting such machine-specific details in the vmcoreinfo, but to unify the implementations across user-land and archs, perhaps this would be good starting point to start a discussion. [0]. https://www.mail-archive.com/[email protected]/msg20300.html [1]. MAN pages -> MAKEDUMPFILE(8) and CRASH(8) [2]. https://www.spinics.net/lists/kexec/msg21608.html http://lists.infradead.org/pipermail/kexec/2018-October/021725.html [3]. https://github.com/bhupesh-sharma/makedumpfile/tree/add-page-offset-base-to-vmcore-v1 Cc: Boris Petkov <[email protected]> Cc: Baoquan He <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Kazuhito Hagio <[email protected]> Cc: Dave Anderson <[email protected]> Cc: James Morse <[email protected]> Cc: Omar Sandoval <[email protected]> Cc: [email protected] Cc: [email protected] Cc: [email protected] Signed-off-by: Bhupesh Sharma <[email protected]> --- arch/x86/kernel/machine_kexec_64.c | 1 + 1 file changed, 1 insertion(+) diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c index 4c8acdfdc5a7..834ccefef867 100644 --- a/arch/x86/kernel/machine_kexec_64.c +++ b/arch/x86/kernel/machine_kexec_64.c @@ -356,6 +356,7 @@ void arch_crash_save_vmcoreinfo(void) VMCOREINFO_SYMBOL(init_top_pgt); vmcoreinfo_append_str("NUMBER(pgtable_l5_enabled)=%d\n", pgtable_l5_enabled()); + VMCOREINFO_NUMBER(page_offset_base); #ifdef CONFIG_NUMA VMCOREINFO_SYMBOL(node_data); -- 2.7.4 _______________________________________________ kexec mailing list [email protected] http://lists.infradead.org/mailman/listinfo/kexec
