Date: Wed, 09 Oct 2013 19:04:26 +0900 From: HATAYAMA Daisuke <d.hatayama@...fujitsu.com> To: Dave Anderson <anderson@...hat.com> CC: Kees Cook <keescook@...omium.org>, LKML <linux-kernel@...r.kernel.org>, x86@...nel.org, kernel-hardening@...ts.openwall.com, Aaron Durbin <adurbin@...gle.com>, Eric Northup <digitaleric@...gle.com>, Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...gle.com>, Mathias Krause <minipli@...glemail.com>, Zhang Yanfei <zhangyanfei@...fujitsu.com>, "H. Peter Anvin" <hpa@...or.com>, "Discussion list for crash utility usage, maintenance and development" <crash-utility@...hat.com> Subject: Re: [PATCH 6/7] x86, kaslr: report kernel offset on panic (2013/10/08 22:38), Dave Anderson wrote: > > > ----- Original Message ----- >> (2013/10/07 22:21), Dave Anderson wrote: >> >>> ----- Original Message ----- >>>> (2013/10/03 22:47), Dave Anderson wrote: >> >>>>> ----- Original Message ----- >>>>>> (2013/10/02 18:13), HATAYAMA Daisuke wrote: >>>>>>> (2013/10/02 16:48), Kees Cook wrote: >> >>>> >>>> Thanks for detailed explanation. So, there's already a feature in crash >>>> utility >>>> to address relocation!, though it's better for me to try them to check if >>>> it's >>>> really applicable to this feature. My concern is whether --reloc works >>>> well >>>> on x86_64 too, because relocation has never done on x86_64 ever, right? >>> >>> Correct. >>> >>>> Another concern is that in case of relocation, users need to additional information >>>> regarding runtime symbol information to crash utility. I want to avoid additional >>>> process, automation is preferable if possible. >>> >>> Right. As I mentioned in the case of 32-bit x86 dumpfiles, there is no automation >>> available when CONFIG_PHYSICAL_START is larger than CONFIG_PHYSICAL_ALIGN. The user >>> either has to be aware of their values in order to calculate the --reloc argument, >>> or has to capture a copy of the /proc/kallsyms file on the crashed system. Typically >>> users/distros using kdump changed their x86 configurations to avoid having to deal >>> with that. >>> >> >> Sorry, I don't understand why relocation size cannot be calculated when >> CONFIG_PHYSICALSTART > CONFIG_PHYSICAL_ALIGN. Could you explain that? > > I just meant that when CONFIG_PHYSICAL_START > CONFIG_PHYSICAL_ALIGN, the 32-bit x86 kernel > gets relocated (like the secondary kdump kernel), but that information is not readily available > from the vmlinux/vmcore pair. > My understanding on CONFIG_PHYSICAL_ALIGN was that starting address of kernel text area is always rounded up to CONFIG_PHYSICAL_ALIGN, only. Your explanation would be part I don't understand well. I'll reconsider it locally... >> >>>> I guess it's enough if there's runtime symbol addresses because we can get relocated >>>> offset value by comparing it with the compile-time symbol address contained in >>>> a given debuginfo file. Candidates for such symbols are the ones contained in >>>> VMCOREINFO note containing some symbol values for makedumpfile to refer to mm-related >>>> objects in kernel, which is always contained in vmcore generated by current kdump and >>>> also vmcores converted by makedumpfile from it. How about this idea? >>> >>> But how would that differ from using an incorrect (non-matching) vmlinux file? >>> >> >> It seems to me almost similar to what crash currently does even if we do relocation check. >> The current check crash currently does is trial-and-error since there's no information >> indicating given vmcore and vmlinuxcertainly match well. >> >> For example, the process I imagine is: >> >> 1) try to match vmcore and vmlinux with no relocation. If fails, go to 2). >> 2) try to match vmcore and vmlinux with relocation. >> >> The two steps include symbol table initialization so it might actually be difficult to >> resume back from 2) to 1). >> >> Also, if gap due to phys_base and gap due to relocation can happen at the same time, >> calculating two values automatically might be futher complicated. So, it would be better >> to add relocation value in VMCOREINFO. Then, what crash utility sholud do becomes very simple. > > Yes please... > > And while you're at it, the kernel's > > VMCOREINFO_SYMBOL(phys_base); > > is pretty much useless, at least w/respect to ELF vmcores, since we need to know its > value in order to translate the address. And I don't think that makedumpfile uses > it when it calculates the phys_base that it stores in compressed kdump headers. Why > not put its value instead of its address? > Yes, I've also noticed this fact. Anyway, I'll post a patch to improvement this phys_base and a patch to export relocation information in VMCOREINFO. -- Thanks. HATAYAMA, Daisuke
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