Re: [Crash-utility] [PATCH] Obtain KASLR offset from early S390X dumps

----- Original Message ----- > If the kernel crashes before vmcoreinfo initialization, there is > no way to extract KASLR offset for such early s390 dumps. > With a new s390 kernel patch, the KASLR offset will be stored in the > lowcore > memory during early boot and then overwritten after vmcoreinfo is > initialized. > This patch allows crash to identify the KASLR offset stored in lowcore > memory for s390 dumps. > > Signed-off-by: Mikhail Zaslonko <zaslonko(a)linux.ibm.com&gt; > --- > s390x.c | 21 +++++++++++++++++++++ > 1 file changed, 21 insertions(+) > > diff --git a/s390x.c b/s390x.c > index 4a1a466..d2c6702 100644 > --- a/s390x.c > +++ b/s390x.c > @@ -46,6 +46,8 @@ > > #define S390X_PSW_MASK_PSTATE 0x0001000000000000UL > > +#define S390X_LC_VMCORE_INFO 0xe0c > + > /* > * Flags for Region and Segment table entries. > */ > @@ -460,6 +462,8 @@ static void s390x_check_live(void) > void > s390x_init(int when) > { > + ulong s390x_lc_kaslr; > + > switch (when) > { > case SETUP_ENV: > @@ -486,6 +490,23 @@ s390x_init(int when) > machdep->verify_paddr = generic_verify_paddr; > machdep->get_kvaddr_ranges = s390x_get_kvaddr_ranges; > machdep->ptrs_per_pgd = PTRS_PER_PGD; > + if (!(kt->flags & RELOC_SET)) { > + /* Read the value from well-known lowcore location*/ > + readmem(S390X_LC_VMCORE_INFO, PHYSADDR, &s390x_lc_kaslr, > + sizeof(s390x_lc_kaslr), "s390x_lc_kaslr", > + FAULT_ON_ERROR); > + /* Check for explicit kaslr offset flag */ > + if (s390x_lc_kaslr & 0x1UL) { > + /* Drop the last bit to get an offset value */ > + s390x_lc_kaslr &= ~(0x1UL); > + /* Make sure that the offset is aligned by 0x1000 */ > + if (s390x_lc_kaslr && !(s390x_lc_kaslr & 0xfff)) { > + kt->relocate = s390x_lc_kaslr * (-1); > + kt->flags |= RELOC_SET; > + kt->flags2 |= KASLR; > + } > + } > + } > break; > > case PRE_GDB: > -- Hi Mikhail, Your patch fails on a live system that utilizes /proc/kcore as the memory source: # ./crash crash 7.2.7++ Copyright (C) 2002-2019 Red Hat, Inc. Copyright (C) 2004, 2005, 2006, 2010 IBM Corporation Copyright (C) 1999-2006 Hewlett-Packard Co Copyright (C) 2005, 2006, 2011, 2012 Fujitsu Limited Copyright (C) 2006, 2007 VA Linux Systems Japan K.K. Copyright (C) 2005, 2011 NEC Corporation Copyright (C) 1999, 2002, 2007 Silicon Graphics, Inc. Copyright (C) 1999, 2000, 2001, 2002 Mission Critical Linux, Inc. This program is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Enter "help copying" to see the conditions. This program has absolutely no warranty. Enter "help warranty" for details. crash: read error: physical address: e0c type: "s390x_lc_kaslr" # That's because the newly-introduced readmem() becomes the very first memory read access, and because you call readmem() with PHYSADDR and FAULT_ON_ERROR, you don't allow crash to pivot from /dev/mem to /proc/kcore when it does its first KVADDR readmem() later on during initialization: # ./crash -d4 ... [ cut ] ... readmem: read_dev_mem() -> /dev/mem <readmem: e0c, PHYSADDR, "s390x_lc_kaslr", 8, (FOE), 3fffbf7e848> <read_dev_mem: addr: e0c paddr: e0c cnt: 8> /dev/mem: Operation not permitted crash: read(/dev/mem, e0c, 8): -1 (ffffffffffffffff) crash: read error: physical address: e0c type: "s390x_lc_kaslr" # Also, if there is *ever* a chance that the readmem() could fail to read that physical address from a dumpfile, I would also suggest that you allow it to fail quietly by changing the readmem() flag from FAULT_ON_ERROR to QUIET|RETURN_ON_ERROR like this: /* Read the value from well-known lowcore location*/ if (readmem(S390X_LC_VMCORE_INFO, PHYSADDR, &s390x_lc_kaslr, sizeof(s390x_lc_kaslr), "s390x_lc_kaslr", QUIET|RETURN_ON_ERROR)) { /* Check for explicit kaslr offset flag */ if (s390x_lc_kaslr & 0x1UL) { /* Drop the last bit to get an offset value */ s390x_lc_kaslr &= ~(0x1UL); /* Make sure that the offset is aligned by 0x1000 */ if (s390x_lc_kaslr && !(s390x_lc_kaslr & 0xfff)) { kt->relocate = s390x_lc_kaslr * (-1); kt->flags |= RELOC_SET; kt->flags2 |= KASLR; } } } Thanks, Dave