Maybe some kernel code stuck the direction flag to STD. Maybe better drop a CLD in there. ..
On Wed, Dec 9, 2015 at 4:16 PM, Davide Libenzi <[email protected]> wrote: > Are we using the proper direction? 😀 > As in, CLD/STD ... > > > On Wed, Dec 9, 2015 at 4:10 PM, ron minnich <[email protected]> wrote: > >> I can imagine a case where the address you're using as the source is not >> pointing where you think it is. I've had that problem >> from time to time. >> >> On Wed, Dec 9, 2015 at 4:05 PM Barret Rhoden <[email protected]> >> wrote: >> >>> So here's a nasty bug of some sort. >>> >>> I'm in the middle of doing some changes to backtrace, so we can easily >>> output user backtraces. In the process, I started running into an >>> issue where the backtrace wouldn't make progress, and would stick on >>> the second entry in the BT. it doesn't always happen either. >>> >>> I narrowed it down to these instructions (hacked up a bit, nops and >>> whatnot). >>> >>> ffffffffc2100081: b9 10 00 00 00 mov $0x10,%ecx >>> ffffffffc2100086: 90 nop >>> ffffffffc2100087: 90 nop >>> ffffffffc2100088: 90 nop >>> ffffffffc2100089: 45 31 c0 xor %r8d,%r8d >>> ffffffffc210008c: 4c 89 e7 mov %r12,%rdi >>> ffffffffc210008f: 4c 89 fe mov %r15,%rsi >>> ffffffffc2100092: f3 a4 rep movsb >>> %ds:(%rsi),%es:(%rdi) >>> ffffffffc2100094: 90 nop >>> ffffffffc2100095: 90 nop >>> ffffffffc2100096: 90 nop >>> ffffffffc2100097: 4c 8b 4d c0 mov -0x40(%rbp),%r9 >>> ffffffffc210009b: 4d 39 f9 cmp %r15,%r9 >>> ffffffffc210009e: 74 30 je ffffffffc21000d0 >>> <backtrace_user_list+0x80> >>> >>> >>> that last bit is a jump to a while(1) loop, so i can look at things in >>> qemu. the check was whether or not some stack variable changed (which >>> we copied into), its addr is rdi == r12. the value should change >>> (based on the program). and when we look at the state of the machine, >>> it's not clear why it didn't. other than r9 and flags from the cmp, >>> our state should be the same as it was right after the rep movsb. >>> >>> (qemu) info registers >>> RAX=ffff80013eb5adf0 RBX=0000000000000001 RCX=0000000000000000 >>> RDX=ffff80013eb5adf0 >>> RSI=00007f7fffbfef50 RDI=ffff80013eb5ada0 RBP=ffff80013eb5ade0 >>> RSP=ffff80013eb5ad80 >>> R8 =0000000000000000 R9 =00007f7fffbfef50 R10=ffff8000000b8f00 >>> R11=ffff8000000b8ec0 >>> R12=ffff80013eb5ada0 R13=0000000000000014 R14=0000000000401a86 >>> R15=00007f7fffbfef50 >>> RIP=ffffffffc21000d0 RFL=00000246 [---Z-P-] CPL=0 II=0 A20=1 SMM=0 HLT=0 >>> ES =0000 0000000000000000 ffffffff 00c00000 >>> CS =0008 0000000000000000 00000000 00209900 DPL=0 CS64 [--A] >>> SS =0000 0000000000000000 ffffffff 00c00000 >>> DS =0000 0000000000000000 ffffffff 00c00000 >>> FS =0000 00004000005d90c0 ffffffff 00c00000 >>> GS =0000 ffffffffc6c8b7c0 ffffffff 00c00000 >>> LDT=0000 0000000000000000 ffffffff 00c00000 >>> TR =0028 ffffffffc6db4380 00000068 00008b00 DPL=0 TSS64-busy >>> GDT= ffff800000101000 00000037 >>> IDT= ffffffffc6c89f10 00000fff >>> CR0=80010033 CR2=000000000061d000 CR3=000000013ecca000 CR4=000007b0 >>> DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 >>> DR3=0000000000000000 >>> DR6=00000000ffff0ff0 DR7=0000000000000400 >>> EFER=0000000000000501 >>> FCW=037f FSW=0000 [ST=0] FTW=00 MXCSR=00001f80 >>> FPR0=0000000000000000 0000 FPR1=0000000000000000 0000 >>> FPR2=0000000000000000 0000 FPR3=0000000000000000 0000 >>> FPR4=0000000000000000 0000 FPR5=0000000000000000 0000 >>> FPR6=0000000000000000 0000 FPR7=0000000000000000 0000 >>> XMM00=0000000000000000ff00000000000000 >>> XMM01=25252525252525252525252525252525 >>> XMM02=00000000000000000000000000000000 >>> XMM03=00000000000000000000000000000000 >>> XMM04=0000000000000000ff00000000000000 >>> XMM05=00000000000000000000000000000000 >>> XMM06=00000000000000000000000000000000 >>> XMM07=00000000000000000000000000000000 >>> XMM08=00000000000000000000000000000000 >>> XMM09=ffffffffffffff00ffffffffffffff00 >>> XMM10=ffffffffffffff00ffffffffffffff00 >>> XMM11=ffffffffffffffffffffffffffffff00 >>> XMM12=00000000000000000000000000000000 >>> XMM13=00000000000000000000000000000000 >>> XMM14=00000000000000000000000000000000 >>> XMM15=00000000000000000000000000000000 >>> >>> note that : >>> >>> RDI = R12 = the destination of the rep movsb. >>> RSI = R15 = the source of the rep movsb >>> RCX = 0 (it was 16), meaning that we did our reps. >>> >>> Here's the destination hexdump: >>> (qemu) x /32wx 0xffff80013eb5ada0 >>> ffff80013eb5ada0: 0xffbfef50 0x00007f7f 0x00401a86 0x00000000 >>> ffff80013eb5adb0: 0x3eb5ade0 0xffff8001 0x00000000 0x00000000 >>> ffff80013eb5adc0: 0x00000000 0x00000000 0x3eb5af40 0xffff8001 >>> ffff80013eb5add0: 0xc6c8b7c0 0xffffffff 0x00401990 0x00000000 >>> ffff80013eb5ade0: 0x3eb5aeb0 0xffff8001 0xc200d341 0xffffffff >>> ffff80013eb5adf0: 0x00402f59 0x00000000 0x00000000 0x00000000 >>> ffff80013eb5ae00: 0x80000001 0x00000000 0x3eb5af40 0xffff8001 >>> ffff80013eb5ae10: 0x000003d4 0x00000000 0x00003ab1 0x00000000 >>> >>> Here's the source hexdump: >>> (qemu) x /32wx 0x00007f7fffbfef50 >>> 00007f7fffbfef50: 0x00000000 0x00000000 0x004020c1 0x00000000 >>> 00007f7fffbfef60: 0xffbfef68 0x00007f7f 0x0000001c 0x00000000 >>> 00007f7fffbfef70: 0x00000001 0x00000000 0xffbfefe8 0x00007f7f >>> 00007f7fffbfef80: 0x00000000 0x00000000 0xffbfeff5 0x00007f7f >>> 00007f7fffbfef90: 0x00000000 0x00000000 0x00000003 0x00000000 >>> 00007f7fffbfefa0: 0x00400040 0x00000000 0x00000004 0x00000000 >>> 00007f7fffbfefb0: 0x00000020 0x00000000 0x00000005 0x00000000 >>> 00007f7fffbfefc0: 0x00000009 0x00000000 0x00000009 0x00000000 >>> >>> >>> the first 16 bytes should be the same. had we actually copied the src >>> into the dst, then the program (backtrace) would work. but it looks >>> like we just silently ignored it. >>> >>> note this is a hacked up copy_from_user(), where it is just a >>> __user_memcpy(), which is what happens when you do a count of say 16 >>> bytes (in this case). >>> >>> you can see from r8 == 0 that there was no error. i also had a printk >>> in the try_exception_fixup just in case. >>> >>> does anyone know of a reason why rep movsb might not work? it sounds >>> crazy. (i also tested on hardware, and it seems to do the same, though >>> i can't inspect the state easily). >>> >>> likewise, there's probably something i'm doing wrong. also note that >>> this code runs from IRQ context (CTRL-B backspace). >>> >>> a super-nasty commit with all of my debugging crap is at >>> origin/nasty-bug if anyone wants to take a look. (i also do an ash >>> ifconfig and epoll_server, which is just some crap to get the user to >>> spin somewhere with a bit of a stack). >>> >>> barret >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "Akaros" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to [email protected]. >>> To post to this group, send email to [email protected]. >>> For more options, visit https://groups.google.com/d/optout. >>> >> -- >> You received this message because you are subscribed to the Google Groups >> "Akaros" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To post to this group, send email to [email protected]. >> For more options, visit https://groups.google.com/d/optout. >> > > -- You received this message because you are subscribed to the Google Groups "Akaros" group. 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