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| author | Christian Krinitsin <mail@krinitsin.com> | 2025-07-17 09:10:43 +0200 |
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| committer | Christian Krinitsin <mail@krinitsin.com> | 2025-07-17 09:10:43 +0200 |
| commit | f2ec263023649e596c5076df32c2d328bc9393d2 (patch) | |
| tree | 5dd86caab46e552bd2e62bf9c4fb1a7504a44db4 /results/scraper/fex/3204 | |
| parent | 63d2e9d409831aa8582787234cae4741847504b7 (diff) | |
| download | qemu-analysis-main.tar.gz qemu-analysis-main.zip | |
Diffstat (limited to 'results/scraper/fex/3204')
| -rw-r--r-- | results/scraper/fex/3204 | 63 |
1 files changed, 63 insertions, 0 deletions
diff --git a/results/scraper/fex/3204 b/results/scraper/fex/3204 new file mode 100644 index 000000000..aa4067e5f --- /dev/null +++ b/results/scraper/fex/3204 @@ -0,0 +1,63 @@ +openat() doesn't ignore dirfd if path is absolute +I'm using FEX 2310, locally built from source according to https://github.com/FEX-Emu/FEX#building-fex, on a Raspberry Pi 4b running the Debian-12-based Raspberry Pi OS; FEX is using an Ubuntu 22.04 rootfs. + +## Steps to reproduce + +Compile this for x86, copy it to the target device and run it: + +``` +#define _GNU_SOURCE +#include <assert.h> +#include <fcntl.h> +#include <stdio.h> +#include <stdlib.h> +#include <unistd.h> + +int main (void) +{ + int res = chdir ("/"); + assert (res == 0); + + int root = openat (AT_FDCWD, "/", O_RDONLY | O_NONBLOCK | O_DIRECTORY | O_CLOEXEC); + printf ("root: %d\n", root); + int fd_abs = openat (root, "/etc/os-release", O_RDONLY | O_CLOEXEC | O_NOCTTY); + printf ("fd,abs: %d\n", fd_abs); + int cwd_abs = openat (AT_FDCWD, "/etc/os-release", O_RDONLY | O_CLOEXEC | O_NOCTTY); + printf ("cwd,abs: %d\n", cwd_abs); + int fd_rel = openat (root, "etc/os-release", O_RDONLY | O_CLOEXEC | O_NOCTTY); + printf ("fd,rel: %d\n", fd_rel); + int cwd_rel = openat (AT_FDCWD, "etc/os-release", O_RDONLY | O_CLOEXEC | O_NOCTTY); + printf ("cwd,rel: %d\n", cwd_rel); + + char *s = NULL; + res = asprintf (&s, "ls -l /proc/%d/fd", getpid ()); + assert (res > 0); + system (s); + return 0; +} +``` + +## Expected result + +The fds identified as `fd,abs` (short for "dirfd = root, path is absolute") and `cwd,abs` (dirfd = AT_FDCWD, path is absolute) point to the `/etc/os-release` from the Ubuntu rootfs, the same one you'd read by `FEXBash -c 'cat /etc/os-release'`. In my case this is really `/home/user/.fex-emu/RootFS/Ubuntu_22_04/usr/lib/os-release`, and `/proc` shows it as such. + +The fds identified as `fd,rel` (dirfd = root, path is relative) and `cwd,rel` (dirfd = AT_FDCWD, path is relative) point to the `/etc/os-release` from the real rootfs, the same one you'd read by `cat /etc/os-release` from an ordinary ARM shell. In my case this is really `/usr/lib/os-release`. + +## Actual result + +The fd identified as `fd,abs` is the real /etc/os-release: FEX is not redirecting it to the rootfs' /etc/os-release. This is not as I expected: the openat(2) man page documents "If the pathname given in pathname is absolute, then dirfd is ignored" and I expected the same thing to be true when using FEX. + +The other three are as I expected. + +## Context + +The `fd,abs` case is a simplification of something I was trying to do in a development branch of Steam's steam-runtime-system-info and pressure-vessel, to make sure that we take FEX's rootfs pseudo-overlay into account when reading information about the real root filesystem. + +## Workaround + +``` +if (path[0] == '/') + fd = AT_FDCWD; +``` + +to force all accesses to absolute paths onto the `cwd,abs` code path, which FEX implements the way I had expected. \ No newline at end of file |