Tweak definition of probestack functions
It looks like the old `__rust_probestack` routine is incompatible with newer linux kernels. My best guess for this is that the kernel's auto-growth logic is failing to trigger, causing what looks like a legitimate segfault to get delivered. My best guess for why *that's* happening is that the faulting address is below `%rsp`, whereas previously all faulting stack addresses were above `%rsp`. The probestack routine does not modify `%rsp` as it's probing the stack, and presumably newer kernels are interpreting this as a legitimate violation. This commit tweaks the probestack routine to instead update `%rsp` incrementally as probing happens. The ABI of the function, however, requires that `%rsp` isn't changed as part of the function so it's restored at the end to the previous value.
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@ -53,28 +53,33 @@ pub unsafe extern fn __rust_probestack() {
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// The ABI here is that the stack frame size is located in `%eax`. Upon
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// The ABI here is that the stack frame size is located in `%eax`. Upon
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// return we're not supposed to modify `%esp` or `%eax`.
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// return we're not supposed to modify `%esp` or `%eax`.
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asm!("
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asm!("
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lea 8(%rsp),%r11 // rsp before calling this routine -> r11
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mov %rax,%r11 // duplicate %rax as we're clobbering %r11
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// Main loop, taken in one page increments. We're decrementing r11 by
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// Main loop, taken in one page increments. We're decrementing rsp by
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// a page each time until there's less than a page remaining. We're
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// a page each time until there's less than a page remaining. We're
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// guaranteed that this function isn't called unless there's more than a
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// guaranteed that this function isn't called unless there's more than a
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// page needed
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// page needed.
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//
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// Note that we're also testing against `8(%rsp)` to account for the 8
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// bytes pushed on the stack orginally with our return address. Using
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// `8(%rsp)` simulates us testing the stack pointer in the caller's
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// context.
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2:
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2:
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sub $$0x1000,%rsp
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test %rsp,8(%rsp)
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sub $$0x1000,%r11
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sub $$0x1000,%r11
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test %r11,(%r11)
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cmp $$0x1000,%r11
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sub $$0x1000,%rax
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cmp $$0x1000,%rax
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ja 2b
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ja 2b
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// Finish up the last remaining stack space requested, getting the last
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// Finish up the last remaining stack space requested, getting the last
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// bits out of rax
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// bits out of r11
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sub %rax,%r11
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sub %r11,%rsp
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test %r11,(%r11)
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test %rsp,8(%rsp)
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// We now know that %r11 is (%rsp + 8 - %rax) so to recover rax
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// Restore the stack pointer to what it previously was when entering
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// we calculate (%rsp + 8) - %r11 which will give us %rax
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// this function. The caller will readjust the stack pointer after we
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lea 8(%rsp),%rax
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// return.
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sub %r11,%rax
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add %rax,%rsp
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ret
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ret
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");
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");
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@ -92,19 +97,18 @@ pub unsafe extern fn __rust_probestack() {
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// The ABI here is the same as x86_64, except everything is 32-bits large.
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// The ABI here is the same as x86_64, except everything is 32-bits large.
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asm!("
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asm!("
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push %ecx
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push %ecx
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lea 8(%esp),%ecx
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mov %eax,%ecx
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2:
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2:
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sub $$0x1000,%esp
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test %esp,8(%esp)
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sub $$0x1000,%ecx
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sub $$0x1000,%ecx
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test %ecx,(%ecx)
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cmp $$0x1000,%ecx
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sub $$0x1000,%eax
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cmp $$0x1000,%eax
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ja 2b
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ja 2b
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sub %eax,%ecx
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sub %ecx,%esp
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test %ecx,(%ecx)
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test %esp,8(%esp)
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lea 8(%esp),%eax
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add %eax,%esp
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sub %ecx,%eax
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pop %ecx
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pop %ecx
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ret
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ret
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");
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");
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