Thanks, I've updated the description accordingly.
** Description changed:
+ [Impact]
+
+ qemu static binaries disadvantageously get their binfmt rule created
+ without the fix-binary flag. This breaks cross-architecture dockers,
+ flatpak-builder, chroots and generally most use cases that entail kernel
+ namespaces or chroots. By default the binfmt module will only open the
+ interpreter file (e.g. qemu-arm-static for armhf binaries) when the
+ kernel actually encounters a file for which the interpreter is needed.
+ This is generally fine, except when the root at the time the file is
+ encountered is no longer the root of the system (IOW: if / is in fact
+ /foo on the system then the interpreter will effectively be looked for
+ in /foo/usr/bin/qemu-arm-static, where it of course does not exist).
+ This is generally fine for regular interpreters such as python, which
+ you want to install inside the namespace/chroot if you want to interpret
+ a python file. Since in qemu's case the interpreter is actually a static
+ emulator it really must come from the host system and not the
+ namespace/chroot. No root file system ever comes with an emulator on
+ board, much less one suitable for the host system.
+
+ As this effectively breaks qemu-based container-use the missing flag
+ really should get rectified. For example you can't easily cross compile
+ for arm inside a chroot. Primary example of this is flatpak-builder
+ which is used to build flatpak bundles. It has cross compilation
+ capabilities expecting a suitable binfmt setup which will fail with
+ obscure execvp errors (on account of not being able to open the
+ interpreter/qemu binary inside the chroot). https://docs.plasma-
+ mobile.org/AppDevelopment.html#creating-a-flatpak-for-the-phone
+
+ The fix-binary flag of binfmt is meant to specifically deal with this.
+ The interpreter file (e.g. qemu-arm-static) is loaded when its binfmt
+ rule is installed instead of when a file that requires it is
+ encountered. When the kernel then encounters a file which requires that
+ interpreter it simply execs the already open file descriptor instead of
+ opening a new one (IOW: the kernel already has the correct file
+ descriptor open, so possibly divergent roots no longer play into finding
+ the interpreter thus allowing namespaces/containers/chroots of a foreign
+ architecture to be run like native ones).
+
+ [Test Case]
+
+ Do note that once successfully loaded, the interpreter likely won't need
+ opening again. So, testing generally should be done after a cold-boot to
+ ensure binfmt hasn't already loaded the interpreter.
+
+ $ sudo apt install docker.io qemu-user-static
+ $ sudo docker run --rm -it armhf/ubuntu /bin/sh
+
+ Without fix-binary this gives an error "exec user process caused 'no such
file or directory" (this is referring to the interpreter not being found).
+ With fix-binary this correctly opens a prompt.
+
+ [Regression Potential]
+
+ Since the file descriptor is always held open I guess memory consumption goes
up by a couple bytes. There isn't any opportunity of regression with fix-binary
as it simply changes the interpreter file opening from lazy to instant.
+ There is potential for the actual SRU patch going wrong, which would manifest
in the binfmt rules not getting set up correctly (this would be shown by the
aforementioned test case though; breaking the rule setup would result in no
improvement to the docker run command).
+
+ [Other Info]
+ I also should point out that the exec() still happens inside the
namespace/confinement, so while this essentially loads a file from the host it
does not impair security as any confinement restrictions will apply to
executable all the same, it is merely the binary data blob that comes from the
host.
+
+ -
+
qemu-user-static in 18.04 invokes update-binfmts without --fix-binary
which can break the binfmt emulation when namespaces or chroots are
involved. Specifically this defuncts cross compiling when relying on
binfmt.
Newer versions already set --fix-binary for the static package. It'd be
really grand if this was actually SRU'd though. Currently 18.04's binfmt
for arm can inobviously fail with obscure errors. Notable example is
bubblewrap and flatpak, which will squarely hit this problem 100% of the
time.
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https://bugs.launchpad.net/bugs/1815100
Title:
qemu-user-static needs to binfmt with --fix-binary
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