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Narrow COW-break on thread stacks.

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PiperOrigin-RevId: 393841270
This commit is contained in:
Jamie Liu
2021-08-30 13:28:04 -07:00
committed by gVisor bot
parent 574c6542a5
commit a247e227b1
1 changed files with 29 additions and 12 deletions
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pkg/sentry/mm/pma.go
+29 -12
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@@ -324,20 +324,37 @@ func (mm *MemoryManager) getPMAsInternalLocked(ctx context.Context, vseg vmaIter
panic(fmt.Sprintf("pma %v needs to be copied for writing, but is not readable: %v", pseg.Range(), oldpma))
}
}
// The majority of copy-on-write breaks on executable pages
// come from:
//
// - The ELF loader, which must zero out bytes on the last
// page of each segment after the end of the segment.
//
// - gdb's use of ptrace to insert breakpoints.
//
// Neither of these cases has enough spatial locality to
// benefit from copying nearby pages, so if the vma is
// executable, only copy the pages required.
var copyAR hostarch.AddrRange
if vseg.ValuePtr().effectivePerms.Execute {
if vma := vseg.ValuePtr(); vma.effectivePerms.Execute {
// The majority of copy-on-write breaks on executable
// pages come from:
//
// - The ELF loader, which must zero out bytes on the
// last page of each segment after the end of the
// segment.
//
// - gdb's use of ptrace to insert breakpoints.
//
// Neither of these cases has enough spatial locality
// to benefit from copying nearby pages, so if the vma
// is executable, only copy the pages required.
copyAR = pseg.Range().Intersect(ar)
} else if vma.growsDown {
// In most cases, the new process will not use most of
// its stack before exiting or invoking execve(); it is
// especially unlikely to return very far down its call
// stack, since async-signal-safety concerns in
// multithreaded programs prevent the new process from
// being able to do much. So only copy up to one page
// before and after the pages required.
stackMaskAR := ar
if newStart := stackMaskAR.Start - hostarch.PageSize; newStart < stackMaskAR.Start {
stackMaskAR.Start = newStart
}
if newEnd := stackMaskAR.End + hostarch.PageSize; newEnd > stackMaskAR.End {
stackMaskAR.End = newEnd
}
copyAR = pseg.Range().Intersect(stackMaskAR)
} else {
copyAR = pseg.Range().Intersect(maskAR)
}