This patch is based on the original work by Petros Angelatos [1], which I have updated to the current git master. The following commit message is largely based on Petros' original message.
In order for one to use QEMU user mode emulation under a chroot, it is required to use binfmt_misc. This can be avoided by QEMU never doing a raw execve() to the host system. Introduce a new option, --execve, that uses the current QEMU interpreter to intercept execve(). qemu_execve() will prepend the interpreter path, similar to what binfmt_misc would do, and then pass the modified execve() to the host. [1] https://patchwork.ozlabs.org/patch/582756/ Signed-off-by: Ricardo Jesus <rj.bcje...@gmail.com> --- linux-user/main.c | 8 +++ linux-user/qemu.h | 1 + linux-user/syscall.c | 138 +++++++++++++++++++++++++++++++++++++++---- 3 files changed, 136 insertions(+), 11 deletions(-) diff --git a/linux-user/main.c b/linux-user/main.c index 75c9785157..52f6860b45 100644 --- a/linux-user/main.c +++ b/linux-user/main.c @@ -107,6 +107,7 @@ static void usage(int exitcode); static const char *interp_prefix = CONFIG_QEMU_INTERP_PREFIX; const char *qemu_uname_release; +const char *qemu_execve_path; /* XXX: on x86 MAP_GROWSDOWN only works if ESP <= address + 32, so we allocate a bigger stack. Need a better solution, for example @@ -337,6 +338,11 @@ static void handle_arg_guest_base(const char *arg) have_guest_base = true; } +static void handle_arg_execve(const char *arg) +{ + qemu_execve_path = strdup(arg); +} + static void handle_arg_reserved_va(const char *arg) { char *p; @@ -441,6 +447,8 @@ static const struct qemu_argument arg_table[] = { "uname", "set qemu uname release string to 'uname'"}, {"B", "QEMU_GUEST_BASE", true, handle_arg_guest_base, "address", "set guest_base address to 'address'"}, + {"execve", "QEMU_EXECVE", true, handle_arg_execve, + "", "use this interpreter when a process calls execve()"}, {"R", "QEMU_RESERVED_VA", true, handle_arg_reserved_va, "size", "reserve 'size' bytes for guest virtual address space"}, {"d", "QEMU_LOG", true, handle_arg_log, diff --git a/linux-user/qemu.h b/linux-user/qemu.h index 5c964389c1..d1b0d5716b 100644 --- a/linux-user/qemu.h +++ b/linux-user/qemu.h @@ -160,6 +160,7 @@ void init_task_state(TaskState *ts); void task_settid(TaskState *); void stop_all_tasks(void); extern const char *qemu_uname_release; +extern const char *qemu_execve_path; extern unsigned long mmap_min_addr; /* ??? See if we can avoid exposing so much of the loader internals. */ diff --git a/linux-user/syscall.c b/linux-user/syscall.c index 945fc25279..a266895a61 100644 --- a/linux-user/syscall.c +++ b/linux-user/syscall.c @@ -115,6 +115,7 @@ #ifdef HAVE_DRM_H #include <libdrm/drm.h> #endif +#include <linux/binfmts.h> #include "linux_loop.h" #include "uname.h" @@ -7674,6 +7675,131 @@ static target_timer_t get_timer_id(abi_long arg) return timerid; } +/* qemu_execve() Must return target values and target errnos. + * + * Although execve() is not an interruptible syscall it is + * a special case where we must use the safe_syscall wrapper: + * if we allow a signal to happen before we make the host + * syscall then we will 'lose' it, because at the point of + * execve the process leaves QEMU's control. So we use the + * safe syscall wrapper to ensure that we either take the + * signal as a guest signal, or else it does not happen + * before the execve completes and makes it the other + * program's problem. + */ +static abi_long qemu_execve(char *filename, char *argv[], + char *envp[]) +{ + char *i_arg = NULL, *i_name = NULL; + char **new_argp; + int argc, fd, ret, i, offset = 5; + char *cp; + char buf[BINPRM_BUF_SIZE]; + + /* normal execve case */ + if (qemu_execve_path == NULL || *qemu_execve_path == 0) { + return get_errno(safe_execve(filename, argv, envp)); + } + + for (argc = 0; argv[argc] != NULL; argc++) { + /* nothing */ ; + } + + fd = open(filename, O_RDONLY); + if (fd == -1) { + return get_errno(fd); + } + + ret = read(fd, buf, BINPRM_BUF_SIZE); + if (ret == -1) { + close(fd); + return get_errno(ret); + } + + /* if we have less than 2 bytes, we can guess it is not executable */ + if (ret < 2) { + close(fd); + return -host_to_target_errno(ENOEXEC); + } + + close(fd); + + /* adapted from the kernel + * https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/fs/binfmt_script.c + */ + if ((buf[0] == '#') && (buf[1] == '!')) { + /* + * This section does the #! interpretation. + * Sorta complicated, but hopefully it will work. -TYT + */ + + buf[BINPRM_BUF_SIZE - 1] = '\0'; + cp = strchr(buf, '\n'); + if (cp == NULL) { + cp = buf + BINPRM_BUF_SIZE - 1; + } + *cp = '\0'; + while (cp > buf) { + cp--; + if ((*cp == ' ') || (*cp == '\t')) { + *cp = '\0'; + } else { + break; + } + } + for (cp = buf + 2; (*cp == ' ') || (*cp == '\t'); cp++) { + /* nothing */ ; + } + if (*cp == '\0') { + return -ENOEXEC; /* No interpreter name found */ + } + i_name = cp; + i_arg = NULL; + for ( ; *cp && (*cp != ' ') && (*cp != '\t'); cp++) { + /* nothing */ ; + } + while ((*cp == ' ') || (*cp == '\t')) { + *cp++ = '\0'; + } + if (*cp) { + i_arg = cp; + } + + if (i_arg) { + offset += 2; + } else { + offset += 1; + } + } + + new_argp = alloca((argc + offset + 1) * sizeof(void *)); + + /* Copy the original arguments with offset */ + for (i = 0; i < argc; i++) { + new_argp[i + offset] = argv[i]; + } + + new_argp[0] = strdup(qemu_execve_path); + new_argp[1] = strdup("--execve"); + new_argp[2] = strdup(qemu_execve_path); + new_argp[3] = strdup("-0"); + new_argp[offset] = filename; + new_argp[argc + offset] = NULL; + + if (i_name) { + new_argp[4] = i_name; + new_argp[5] = i_name; + + if (i_arg) { + new_argp[6] = i_arg; + } + } else { + new_argp[4] = argv[0]; + } + + return get_errno(safe_execve(qemu_execve_path, new_argp, envp)); +} + static int target_to_host_cpu_mask(unsigned long *host_mask, size_t host_size, abi_ulong target_addr, @@ -8023,17 +8149,7 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1, if (!(p = lock_user_string(arg1))) goto execve_efault; - /* Although execve() is not an interruptible syscall it is - * a special case where we must use the safe_syscall wrapper: - * if we allow a signal to happen before we make the host - * syscall then we will 'lose' it, because at the point of - * execve the process leaves QEMU's control. So we use the - * safe syscall wrapper to ensure that we either take the - * signal as a guest signal, or else it does not happen - * before the execve completes and makes it the other - * program's problem. - */ - ret = get_errno(safe_execve(p, argp, envp)); + ret = qemu_execve(p, argp, envp); unlock_user(p, arg1, 0); goto execve_end; -- 2.27.0