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runtime: handle sysReserve returning a pointer below the arena
In mheap.sysAlloc, if an allocation at arena_used would exceed arena_end (but wouldn't yet push us past arena_start+_MaxArean32), it trie to extend the arena reservation by another 256 MB. It extends the arena by calling sysReserve, which, on 32-bit, calls mmap without MAP_FIXED, which means the address is just a hint and the kernel can put the mapping wherever it wants. In particular, mmap may choose an address below arena_start (the kernel also chose arena_start, so there could be lots of space below it). Currently, we don't detect this case and, if it happens, mheap.sysAlloc will corrupt arena_end and arena_used then return the low pointer to mheap.grow, which will crash when it attempts to index in to h_spans with an underflowed index. Fix this by checking not only that that p+p_size isn't too high, but that p isn't too low. Fixes #13143. Change-Id: I8d0f42bd1484460282a83c6f1a6f8f0df7fb2048 Reviewed-on: https://go-review.googlesource.com/16927 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>
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@ -404,7 +404,7 @@ func (h *mheap) sysAlloc(n uintptr) unsafe.Pointer {
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if p == h.arena_end {
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h.arena_end = new_end
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h.arena_reserved = reserved
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} else if p+p_size <= h.arena_start+_MaxArena32 {
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} else if h.arena_start <= p && p+p_size <= h.arena_start+_MaxArena32 {
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// Keep everything page-aligned.
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// Our pages are bigger than hardware pages.
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h.arena_end = p + p_size
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