On 12 January 2017 at 04:05, Richard Henderson <r...@twiddle.net> wrote:
> If the interp_prefix is a complete chroot, it may have a *lot* of files.
> Setting up the cache for this is quite expensive. Instead, use the *at
> versions of various syscalls to attempt the operation in the prefix.
>
> Signed-off-by: Richard Henderson <r...@twiddle.net>
> ---
> linux-user/elfload.c | 12 ++-
> linux-user/main.c | 3 +-
> linux-user/qemu.h | 1 +
> linux-user/syscall.c | 236
> ++++++++++++++++++++++++++++++++++++++++++---------
> util/Makefile.objs | 2 +-
> util/path.c | 178 --------------------------------------
> 6 files changed, 209 insertions(+), 223 deletions(-)
> delete mode 100644 util/path.c
>
> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> index 547053c..8b947fd 100644
> --- a/linux-user/elfload.c
> +++ b/linux-user/elfload.c
> @@ -2026,7 +2026,17 @@ static void load_elf_interp(const char *filename,
> struct image_info *info,
> {
> int fd, retval;
>
> - fd = open(path(filename), O_RDONLY);
> + switch (filename[0]) {
> + case '/':
> + fd = openat(interp_dirfd, filename + 1, O_RDONLY);
> + if (fd >= 0 || errno != ENOENT) {
> + break;
> + }
> + /* fallthru */
> + default:
> + fd = open(filename, O_RDONLY);
> + break;
> + }
> if (fd < 0) {
> goto exit_perror;
> }
> diff --git a/linux-user/main.c b/linux-user/main.c
> index c1d5eb4..dba988b 100644
> --- a/linux-user/main.c
> +++ b/linux-user/main.c
> @@ -81,6 +81,7 @@ unsigned long reserved_va;
> static void usage(int exitcode);
>
> static const char *interp_prefix = CONFIG_QEMU_INTERP_PREFIX;
> +int interp_dirfd;
> const char *qemu_uname_release;
>
> /* XXX: on x86 MAP_GROWSDOWN only works if ESP <= address + 32, so
> @@ -4013,7 +4014,7 @@ int main(int argc, char **argv, char **envp)
> memset(&bprm, 0, sizeof (bprm));
>
> /* Scan interp_prefix dir for replacement files. */
> - init_paths(interp_prefix);
> + interp_dirfd = open(interp_prefix, O_CLOEXEC | O_DIRECTORY | O_PATH);
I've been using this patch over the last week or so as a convenient
way of being able to run guest binaries without having to actually use
chroot, and I just noticed a bug here:
if the interp_prefix doesn't exist, this will set interp_dirfd to -1
and then every file access will fail with EBADF. We should treat "prefix
doesn't exist" like "don't use a prefix", because by default we use
/usr/gnemul/qemu-something, which probably doesn't exist for most people.
thanks
-- PMM
