tree 1bf3d4aaf19d39bf2b8a259abfb6fd4ec1068d45
parent 856daee36f0ee3b8e4bced8517d02ebb0f8846b7
author Miklos Szeredi <mszeredi@redhat.com> 1487346972 +0100
committer Miklos Szeredi <mszeredi@redhat.com> 1487346972 +0100

ovl: intercept mmap on overlay files

... in order to handle the corner case when the file is copied up after
being opened read-only and mapped shared.

Can be verified with the following script:

 - 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - - 
cd /
rm -rf /tmp/ovl-rorw-test
mkdir /tmp/ovl-rorw-test
cd /tmp/ovl-rorw-test
cat << EOF > rorw-map.c
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <err.h>
#include <sys/mman.h>

int main(int argc, char *argv[])
{
        int rofd, rwfd;
        int ret;
        char buf[4];
        char *addr;

        rofd = open(argv[1], O_RDONLY);
        if (rofd == -1)
                err(1, "ro open");

        addr = mmap(NULL, 4, PROT_READ, MAP_SHARED, rofd, 0);
        if (addr == MAP_FAILED)
                err(1, "ro mmap");

        if (memcmp(addr, "bubu", 4) == 0)
                errx(1, "identical startup data");

        rwfd = open(argv[1], O_WRONLY);
        if (rwfd == -1)
                err(1, "rw open");

        ret = write(rwfd, "bubu", 4);
        if (ret == -1)
                err(1, "write");
        if (ret < 4)
                errx(1, "short write");

        if (memcmp(addr, "bubu", 4) != 0)
                errx(1, "bad mmap data");

        ret = read(rofd, buf, 4);
        if (ret == -1)
                err(1, "read");
        if (ret < 4)
                errx(1, "short read");
        if (memcmp(buf, "bubu", 4) != 0)
                errx(1, "bad read data");

        return 0;
}
EOF
gcc -o rorw-map rorw-map.c
mkdir -p mnt lower upper work
echo baba > lower/foo
mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work mnt
./rorw-map mnt/foo
umount mnt
 - 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - - 

No output means success, "rorw-map: bad mmap data" means failure.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>