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static linux-user ARM emulation has several-second startup time
static linux-user emulation has several-second startup time
My problem: I'm a Parabola packager, and I'm updating our
qemu-user-static package from 2.8 to 2.11. With my new
statically-linked 2.11, running `qemu-arm /my/arm-chroot/bin/true`
went from taking 0.006s to 3s! This does not happen with the normal
dynamically linked 2.11, or the old static 2.8.
What happens is it gets stuck in
`linux-user/elfload.c:init_guest_space()`. What `init_guest_space`
does is map 2 parts of the address space: `[base, base+guest_size]`
and `[base+0xffff0000, base+0xffff0000+page_size]`; where it must find
an acceptable `base`. Its strategy is to `mmap(NULL, guest_size,
...)` decide where the first range is, and then check if that
+0xffff0000 is also available. If it isn't, then it starts trying
`mmap(base, ...)` for the entire address space from low-address to
high-address.
"Normally," it finds an accaptable `base` within the first 2 tries.
With a static 2.11, it's taking thousands of tries.
----
Now, from my understanding, there are 2 factors working together to
cause that in static 2.11 but not the other builds:
- 2.11 increased the default `guest_size` from 0xf7000000 to 0xffff0000
- PIE (and thus ASLR) is disabled for static builds
For some reason that I don't understand, with the smaller
`guest_size` the initial `mmap(NULL, guest_size, ...)` usually
returns an acceptable address range; but larger `guest_size` makes it
consistently return a block of memory that butts right up against
another already mapped chunk of memory. This isn't just true on the
older builds, it's true with the 2.11 builds if I use the `-R` flag to
shrink the `guest_size` back down to 0xf7000000. That is with
linux-hardened 4.13.13 on x86-64.
So then, it it falls back to crawling the entire address space; so it
tries base=0x00001000. With ASLR, that probably succeeds. But with
ASLR being disabled on static builds, the text segment is at
0x60000000; which is does not leave room for the needed
0xffff1000-size block before it. So then it tries base=0x00002000.
And so on, more than 6000 times until it finally gets to and passes
the text segment; calling mmap more than 12000 times.
----
I'm not sure what the fix is. Perhaps try to mmap a continuous chunk
of size 0xffff1000, then munmap it and then mmap the 2 chunks that we
actually need. The disadvantage to that is that it does not support
the sparse address space that the current algorithm supports for
`guest_size < 0xffff0000`. If `guest_size < 0xffff0000` *and* the big
mmap fails, then it could fall back to a sparse search; though I'm not
sure the current algorithm is a good choice for it, as we see in this
bug. Perhaps it should inspect /proc/self/maps to try to find a
suitable range before ever calling mmap?
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