Messages in this thread | | | Subject | Re: [RFC PATCH] x86/mm/fault: Allow stack access below %rsp | From | Waiman Long <> | Date | Mon, 5 Nov 2018 11:27:04 -0500 |
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On 11/02/2018 06:28 PM, Dave Hansen wrote: > On 11/2/18 12:50 PM, Waiman Long wrote: >> On 11/02/2018 03:44 PM, Dave Hansen wrote: >>> On 11/2/18 12:40 PM, Waiman Long wrote: >>>> The 64k+ limit check is kind of arbitrary. So the check is now removed >>>> to just let expand_stack() decide if a segmentation fault should happen. >>> With the 64k check removed, what's the next limit that we bump into? Is >>> it just the stack_guard_gap space above the next-lowest VMA? >> I think it is both the stack_guard_gap space above the next lowest VMA >> and the rlimit(RLIMIT_STACK). > The gap seems to be hundreds of megabytes, typically where RLIMIT_STACK > is 8MB by default, so RLIMIT_STACK is likely to be the practical limit > that will be hit. So, practically, we've taken a ~64k area that we > would on-demand extend the stack into in one go, and turned that into a > the full ~8MB area that you could have expanded into anyway, but all at > once. > > That doesn't seem too insane, especially since we don't physically back > the 8MB or anything. Logically, it also seems like you *should* be able > to touch any bit of the stack within the rlimit. > > But, on the other hand, as our comments say: "Accessing the stack below > %sp is always a bug." Have we been unsuccessful in convincing our gcc > buddies of this?
With gcc 4.4.7, the object code for the sample program in the commit log are:
0x00000000004004c4 <+0>: push %rbp 0x00000000004004c5 <+1>: mov %rsp,%rbp 0x00000000004004c8 <+4>: push %rbx 0x00000000004004c9 <+5>: sub $0x18,%rsp 0x00000000004004cd <+9>: lea -0x2ff8(%rsp),%rax 0x00000000004004d5 <+17>: movq $0x0,(%rax) 0x00000000004004dc <+24>: mov %rsp,%rax 0x00000000004004df <+27>: mov %rax,%rbx 0x00000000004004e2 <+30>: lea -0x3ff8(%rsp),%rax 0x00000000004004ea <+38>: lea -0x43008(%rsp),%rdx 0x00000000004004f2 <+46>: jmp 0x400501 <main+61> 0x00000000004004f4 <+48>: movq $0x0,(%rax) 0x00000000004004fb <+55>: sub $0x1000,%rax 0x0000000000400501 <+61>: cmp %rdx,%rax 0x0000000000400504 <+64>: ja 0x4004f4 <main+48> 0x0000000000400506 <+66>: movq $0x0,(%rdx) 0x000000000040050d <+73>: sub $0x40010,%rsp 0x0000000000400514 <+80>: mov %rsp,%rax 0x0000000000400517 <+83>: add $0xf,%rax 0x000000000040051b <+87>: shr $0x4,%rax 0x000000000040051f <+91>: shl $0x4,%rax 0x0000000000400523 <+95>: mov %rax,-0x18(%rbp) 0x0000000000400527 <+99>: mov $0x400638,%edi 0x000000000040052c <+104>: callq 0x4003b8 <puts@plt> 0x0000000000400531 <+109>: mov $0x0,%eax 0x0000000000400536 <+114>: mov %rbx,%rsp 0x0000000000400539 <+117>: mov -0x8(%rbp),%rbx 0x000000000040053d <+121>: leaveq 0x000000000040053e <+122>: retq
With a newer gcc 4.8.5, the object code becomes
0x000000000040052d <+0>: push %rbp 0x000000000040052e <+1>: mov %rsp,%rbp 0x0000000000400531 <+4>: lea -0x1020(%rsp),%rsp 0x0000000000400539 <+12>: mov $0xfffffffffffc0000,%r11 0x0000000000400540 <+19>: lea (%rsp,%r11,1),%r11 0x0000000000400544 <+23>: cmp %r11,%rsp 0x0000000000400547 <+26>: je 0x400557 <main+42> 0x0000000000400549 <+28>: sub $0x1000,%rsp 0x0000000000400550 <+35>: orq $0x0,(%rsp) 0x0000000000400555 <+40>: jmp 0x400544 <main+23> 0x0000000000400557 <+42>: lea 0x1020(%rsp),%rsp 0x000000000040055f <+50>: mov $0x400600,%edi 0x0000000000400564 <+55>: callq 0x400410 <puts@plt> 0x0000000000400569 <+60>: mov $0x0,%eax 0x000000000040056e <+65>: leaveq 0x000000000040056f <+66>: retq
So gcc had changed to avoid doing that, but my main concern are old binaries that were compiled with old gcc.
Cheers, Longman
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