runInLinuxVM: pass .attrs.sh explicitly instead of whole /build directory

The approach taken in [1] breaks down as soon as vmRunCommand is
manually called with an overriden TMPDIR, like disko does it. /build
will just not be available.

By moving the .attrs.sh file into the xchg folder explicitly, we can all
the "exchange infrastructure" the same as before, thus avoid more
breakage.

This reverts some parts of [1].

[1]: 97ed6b4565

pkgs/build-support/vm/default.nix

@@ -125,15 +125,15 @@ rec {
     mkdir -p /fs${storeDir}
     mount -t 9p store /fs${storeDir} -o trans=virtio,version=9p2000.L,cache=loose,msize=131072
 
-    echo "mounting host's build directory..."
-    mkdir -p /fs/build
-    mount -t 9p sa /fs/build -o trans=virtio,version=9p2000.L,cache=loose,msize=131072
-
     mkdir -p /fs/tmp /fs/run /fs/var
     mount -t tmpfs -o "mode=1777" none /fs/tmp
     mount -t tmpfs -o "mode=755" none /fs/run
     ln -sfn /run /fs/var/run
 
+    echo "mounting host's temporary directory..."
+    mkdir -p /fs/tmp/xchg
+    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L,msize=131072
+
     mkdir -p /fs/proc
     mount -t proc none /fs/proc
 
@@ -165,9 +165,11 @@ rec {
   stage2Init = writeScript "vm-run-stage2" ''
     #! ${bash}/bin/sh
     set -euo pipefail
-    source /build/xchg/saved-env
-    if [ -f "''${NIX_ATTRS_SH_FILE-}" ]; then
-      source "$NIX_ATTRS_SH_FILE"
+    source /tmp/xchg/saved-env
+    if [ -f /tmp/xchg/.attrs.sh ]; then
+      source /tmp/xchg/.attrs.sh
+      export NIX_ATTRS_JSON_FILE=/tmp/xchg/.attrs.json
+      export NIX_ATTRS_SH_FILE=/tmp/xchg/.attrs.sh
     fi
 
     export NIX_STORE=${storeDir}
@@ -177,6 +179,7 @@ rec {
     cd "$NIX_BUILD_TOP"
 
     source $stdenv/setup
+
     if ! test -e /bin/sh; then
       ${coreutils}/bin/mkdir -p /bin
       ${coreutils}/bin/ln -s ${bash}/bin/sh /bin/sh
@@ -201,7 +204,7 @@ rec {
       declare -a argsArray=()
       concatTo argsArray origArgs
       "$origBuilder" "''${argsArray[@]}"
-      echo $? > /build/xchg/in-vm-exit
+      echo $? > /tmp/xchg/in-vm-exit
 
       ${busybox}/bin/mount -o remount,ro dummy /
 
@@ -220,7 +223,7 @@ rec {
       -nographic -no-reboot \
       -device virtio-rng-pci \
       -virtfs local,path=${storeDir},security_model=none,mount_tag=store \
-      -virtfs local,path=/build,security_model=none,mount_tag=sa \
+      -virtfs local,path=xchg,security_model=none,mount_tag=xchg \
       ''${diskImage:+-drive file=$diskImage,if=virtio,cache=unsafe,werror=report} \
       -kernel ${kernel}/${img} \
       -initrd ${initrd}/initrd \
@@ -235,6 +238,7 @@ rec {
     PATH=${coreutils}/bin
 
     if [ -f "''${NIX_ATTRS_SH_FILE-}" ]; then
+      cp $NIX_ATTRS_JSON_FILE $NIX_ATTRS_SH_FILE xchg
       source "$NIX_ATTRS_SH_FILE"
     fi
     source $stdenv/setup