Date: Sat, 29 Apr 2006 08:46:38 +0300 From: Kostik Belousov <kostikbel@gmail.com> To: Kris Kennaway <kris@obsecurity.org> Cc: current@freebsd.org Subject: Re: Page fault from kdb_backtrace() Message-ID: <20060429054638.GC4436@deviant.kiev.zoral.com.ua> In-Reply-To: <20060429050752.GB23613@xor.obsecurity.org> References: <20060429050752.GB23613@xor.obsecurity.org>
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[-- Attachment #1 --] On Sat, Apr 29, 2006 at 01:07:52AM -0400, Kris Kennaway wrote: > I am testing a patch that has the effect of spewing a lot of > kdb_backtraces to the console..but sooner or later it will panic: > > trap 12: page fault while in kernel mode > cpuid = 1; apic id = 06 > fault virtual address = 0xc > fault code = supervisor read, page not present > instruction pointer = 0x20:0xc0466c61 > stack pointer = 0x28:0xf599da20 > frame pointer = 0x28:0xf599da40 > code segment = base 0x0, limit 0xfffff, type 0x1b > = DPL 0, pres 1, def32 1, gran 1 > processor eflags = interrupt enabled, resume, IOPL = 0 > current process = 24902 (mkdir) > [thread pid 24902 tid 100129 ] > Stopped at X_db_symbol_values+0x18: cmpl $0,0xc(%eax) > db> wh > Tracing pid 24902 tid 100129 td 0xca7f4bd0 > X_db_symbol_values(0,c0851b2c,f599da98,f599da58,2f) at X_db_symbol_values+0x18 > db_symbol_values(c0851b2c,f599da98,0,c05b3c8e,c06f8653) at db_symbol_values+0x41 > db_backtrace(ca7f4bd0,0,f599db1c,c055e161,ffffffff) at db_backtrace+0x96 > db_trace_self(c073e687,f599db38,c05b8cd0,c0746e8f,ca7f4bd0) at db_trace_self+0x4d > kdb_backtrace(c0746e8f,ca7f4bd0,0,0,0) at kdb_backtrace+0x2f > vn_finished_write(ca59787c,ffffffdf,c0746c88,d68,0) at vn_finished_write+0x33 > kern_mkdir(ca7f4bd0,bfb623a0,0,1f8,f599dd30) at kern_mkdir+0x3a7 > mkdir(ca7f4bd0,f599dd04,8,445,ccef4528) at mkdir+0x29 > syscall(2805003b,3b,bfbe003b,28051a1a,24c) at syscall+0x307 > Xint0x80_syscall() at Xint0x80_syscall+0x1f > --- syscall (136, FreeBSD ELF32, mkdir), eip = 0x2813d87b, esp = 0xbfb61f3c, ebp = 0xbfb62378 --- > > Why is this happening? > > Kris Is this true that kdb_backtrace called from several threads at parallel ? Symbol lookup seems to be non-SMP safe. It looks like global db_last_symtab is used unsafely. At least, try to comment the line 233 of sys/ddb/db_sym.c, that sets db_last_symtab to NULL. Sorry, I cannot test the change now. [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.3 (FreeBSD) iD8DBQFEUv2+C3+MBN1Mb4gRAqbyAKDrtrbcuCVi2EiQ/EsrO4joUVMGFgCfSbyH KhcOO/3+PvN6kvetiNuMww4= =0xvV -----END PGP SIGNATURE-----help
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