id = 2825
title = "execveat with file descriptor and empty filename returns ENOENT when cross architectures"
state = "opened"
created_at = "2025-02-21T13:01:36.698Z"
closed_at = "n/a"
labels = []
url = "https://gitlab.com/qemu-project/qemu/-/issues/2825"
host-os = "Debian 12"
host-arch = "x86"
qemu-version = "9.2.0"
guest-os = "n/a"
guest-arch = "n/a"
description = """On my x86_64 debian host (with binfmt_misc configured), when calling execveat with a fd , and empty pathname "", and flag AT_EMPTY_PATH. Then only x86_64 and x86 can be called normally, while programs of other architectures (arm64, arm, riscv64, etc.) will return ENOENT errors.

I first encountered this problem when trying to run lxc-attach with qemu-aarch64. Its reference is [lxc/stable-6.0/src/include/fexecve.c#L30](https://github.com/lxc/lxc/blob/stable-6.0/src/include/fexecve.c#L30), which is the implementation of the fexecve function. So I wrote a simple test and compiled it with `x86_64/aarch64-linux-gnu-gcc -static test.c -o test`. execveat works fine when running natively or using qemu-x86_64/qemu-i386. When running versions for other architectures, using AT_EMPTY_PATH will result in ENOENT (No such file or directory); use /proc/self/fd/%d as the pathname and execve, it will work fine (like the rest part of the fexecve function). If binfmt_misc is turned off and run forign architectures ver, both calls will result in ENOEXEC (Exec format error)."""
reproduce = """1. Install qemu-user and binfmt_misc. Install gcc-aarch64-linux-gnu/gcc-riscv64-linux-gnu etc.
2. Compile test.c with host gcc, then compile forign architectures ver with gcc-aarch64-linux-gnu/gcc-riscv64-linux-gnu etc. like `gcc -static test.c -o test` and `aarch64-linux-gnu-gcc -static test.c -o test-aarch64`
3. Run different versions of test
4. To disable/enable binfmt, you can `echo 0 > /proc/sys/fs/binfmt_misc/qemu-aarch64` or `echo 1 > /proc/sys/fs/binfmt_misc/qemu-aarch64`
5. Sample outputs

```
rrex@debian:~/Downloads$ ./test
****Running to prepare execve
fd=3
File size: 772296 bytes

execveat with AT_EMPTY_PATH:
**Running in execve

execveat with fd path: /proc/self/fd/3
**Running in execve

rrex@debian:~/Downloads$ qemu-aarch64 ./test-aarch64 
****Running to prepare execve
fd=3
File size: 706104 bytes

execveat with AT_EMPTY_PATH:
!!execveat a fd failed with errno: No such file or directory

execveat with fd path: /proc/self/fd/3
**Running in execve
```"""
additional = """#"""