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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