commit 7a8d9f3a0e882df50681e40f09c29cfb4966ea2d
author Pierrick Bouvier <pierrick.bouvier@linaro.org> Wed Jul 05 14:10:23 2023 +0200
committer Richard Henderson <richard.henderson@linaro.org> Sat Jul 15 08:02:32 2023 +0100
tree 420c17e6e6ddaef84c89c4e0094eb8982e03cc6b
parent ea9812d93f9c3e1a308ac33097021c50d581d10e

linux-user/syscall: Implement execve without execveat

Support for execveat syscall was implemented in 55bbe4 and is available
since QEMU 8.0.0. It relies on host execveat, which is widely available
on most of Linux kernels today.

However, this change breaks qemu-user self emulation, if "host" qemu
version is less than 8.0.0. Indeed, it does not implement yet execveat.
This strange use case happens with most of distribution today having
binfmt support.

With a concrete failing example:
$ qemu-x86_64-7.2 qemu-x86_64-8.0 /bin/bash -c /bin/ls
/bin/bash: line 1: /bin/ls: Function not implemented
-> not implemented means execve returned ENOSYS

qemu-user-static 7.2 and 8.0 can be conveniently grabbed from debian
packages qemu-user-static* [1].

One usage of this is running wine-arm64 from linux-x64 (details [2]).
This is by updating qemu embedded in docker image that we ran into this
issue.

The solution to update host qemu is not always possible. Either it's
complicated or ask you to recompile it, or simply is not accessible
(GitLab CI, GitHub Actions). Thus, it could be worth to implement execve
without relying on execveat, which is the goal of this patch.

This patch was tested with example presented in this commit message.

[1] http://ftp.us.debian.org/debian/pool/main/q/qemu/
[1] https://www.linaro.org/blog/emulate-windows-on-arm/

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Message-Id: <20230705121023.973284-1-pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/syscall.c

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 420bab7..c15d9ad 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -659,6 +659,7 @@
 #endif
 safe_syscall5(int, waitid, idtype_t, idtype, id_t, id, siginfo_t *, infop, \
               int, options, struct rusage *, rusage)
+safe_syscall3(int, execve, const char *, filename, char **, argv, char **, envp)
 safe_syscall5(int, execveat, int, dirfd, const char *, filename,
               char **, argv, char **, envp, int, flags)
 #if defined(TARGET_NR_select) || defined(TARGET_NR__newselect) || \
@@ -8629,9 +8630,9 @@
     return ret;
 }
 
-static int do_execveat(CPUArchState *cpu_env, int dirfd,
-                       abi_long pathname, abi_long guest_argp,
-                       abi_long guest_envp, int flags)
+static int do_execv(CPUArchState *cpu_env, int dirfd,
+                    abi_long pathname, abi_long guest_argp,
+                    abi_long guest_envp, int flags, bool is_execveat)
 {
     int ret;
     char **argp, **envp;
@@ -8710,11 +8711,14 @@
         goto execve_efault;
     }
 
+    const char *exe = p;
     if (is_proc_myself(p, "exe")) {
-        ret = get_errno(safe_execveat(dirfd, exec_path, argp, envp, flags));
-    } else {
-        ret = get_errno(safe_execveat(dirfd, p, argp, envp, flags));
+        exe = exec_path;
     }
+    ret = is_execveat
+        ? safe_execveat(dirfd, exe, argp, envp, flags)
+        : safe_execve(exe, argp, envp);
+    ret = get_errno(ret);
 
     unlock_user(p, pathname, 0);
 
@@ -9406,9 +9410,9 @@
         return ret;
 #endif
     case TARGET_NR_execveat:
-        return do_execveat(cpu_env, arg1, arg2, arg3, arg4, arg5);
+        return do_execv(cpu_env, arg1, arg2, arg3, arg4, arg5, true);
     case TARGET_NR_execve:
-        return do_execveat(cpu_env, AT_FDCWD, arg1, arg2, arg3, 0);
+        return do_execv(cpu_env, AT_FDCWD, arg1, arg2, arg3, 0, false);
     case TARGET_NR_chdir:
         if (!(p = lock_user_string(arg1)))
             return -TARGET_EFAULT;