Read and Safely Test /proc/sys/abi/vsyscall32

A 32-bit compatibility bug sends you looking at /proc/sys/abi/vsyscall32, so here is how to check it, read it, and test it with a way back. This uses the installed Linux man-pages 6.7 documentation on an Ubuntu 6.8.0-139-generic kernel. Allow about ten minutes: the inspection steps are unprivileged, changing the setting needs elevated privileges.

1. Check whether the ABI directory exists

proc_sys_abi(5) documents /proc/sys/abi/ as a directory that may contain application binary information. It is not guaranteed to exist on every system, so check before naming a file:

$ test -d /proc/sys/abi && printf '%s\n' 'ABI sysctl directory is present' || printf '%s\n' 'ABI sysctl directory is absent'

If it is present, list the files without changing anything:

$ find /proc/sys/abi -maxdepth 1 -type f -printf '%f\n' | sort
vsyscall32

Your list may be empty or may hold different entries. Do not assume a setting mentioned in an online example exists on this kernel.

2. Read the setting directly

Read the file as an ordinary user. The kernel supplies the value followed by a newline:

$ cat /proc/sys/abi/vsyscall32
1

• 1 enables the vDSO page mapped into 32-bit processes.
• 0 disables that mapping.
• Default is enabled when the kernel has CONFIG_COMPAT_VDSO, otherwise disabled.

This is a 32-bit compatibility setting, not a general switch for 64-bit vDSO support. For scripts, the procps sysctl command gives the same value under a name that is easier to fold into status output:

$ sysctl abi.vsyscall32
abi.vsyscall32 = 1
$ sysctl -n abi.vsyscall32
1

Both of those are read-only. The -n form prints only the value, handy for a comparison:

value=$(sysctl -n abi.vsyscall32) || exit 1
case "$value" in
    0|1) printf 'vsyscall32=%s\n' "$value" ;;
    *) printf 'unexpected vsyscall32 value: %s\n' "$value" >&2; exit 2 ;;
esac

3. Relate the file to the kernel boot option

Here is the surprising bit: the upstream ABI documentation says this setting controls the same behaviour as the x86 vdso32 kernel boot parameter. That is why a value can reflect an early boot decision rather than a local application config file. It does not mean writing the sysctl edits the boot loader or changes the next boot.

Check the running kernel and its command line as read-only diagnostics:

$ uname -r
6.8.0-139-generic
$ tr ' ' '\n' < /proc/cmdline | grep '^vdso32$' || printf '%s\n' 'vdso32 is not present on the command line'

An absent boot parameter is not proof of the effective default, because kernel configuration matters too. Treat /proc/sys/abi/vsyscall32 as the value currently in force.

4. Record the old value before testing

Capture the current value in a shell variable, and refuse to continue if it is not the documented binary form:

old_value=$(cat /proc/sys/abi/vsyscall32) || exit 1
case "$old_value" in
    0|1) printf 'recorded old value: %s\n' "$old_value" ;;
    *) printf 'unexpected old value: %s\n' "$old_value" >&2; exit 2 ;;
esac

Now, and only if the test is authorised, write the alternate value with sysctl:

$ sudo sysctl -w abi.vsyscall32=0
abi.vsyscall32 = 0

This is the first command in the guide that normally needs sudo. It writes the live sysctl; it does not create a persistent configuration file. Verify immediately:

$ sysctl -n abi.vsyscall32
0

If the write is rejected, keep the error and investigate the host's permissions and kernel support rather than retrying with different values.

5. Restore the recorded value

Restore the exact value captured before the test. If you opened a new shell, replace OLD_VALUE with the value you recorded, not a guessed default:

$ sudo sysctl -w abi.vsyscall32=OLD_VALUE
abi.vsyscall32 = OLD_VALUE
$ sysctl -n abi.vsyscall32
OLD_VALUE

sudo sysctl -w "abi.vsyscall32=$old_value"
test "$(sysctl -n abi.vsyscall32)" = "$old_value"

There is no service restart or file deletion to undo here: the recovery action is the explicit write back to the recorded value. A reboot may also reapply the kernel's boot-time choice, but do not reboot a shared host as a substitute for restoring and verifying the setting.

Common traps

• Treating it as storage. /proc/sys/abi/ is a kernel interface, not a directory of application binaries or a place to store configuration files.
• Papering over an absence. A missing directory or file is a supported observation; it does not justify creating one with mkdir or placing a similarly named file elsewhere.
• Over-reading a success. vsyscall32=1 describes the mapping setting. It does not prove a 32-bit application starts successfully, that a process uses the page, or that other compatibility interfaces work.
• Assuming a fixed file list. The man page gives the directory's purpose and points to kernel documentation; it does not promise a fixed list of files. Follow the running kernel's exposed interface.
• Confusing runtime and boot. If the setting must survive a reboot, set up the distribution's supported boot or sysctl configuration separately and test its precedence before changing it.