Sony Xperia XA2 backup before installing Sailfish?

that would require finding an exploit to gain root access...

Xperia X could use the DirtyCow exploit, but that only woeked on Android 6 Marshmallow. It won't work on the more recent Androids of the Xa2

There are some people on XDA-developpers forum who have managed to exploit a Sony Xperia XZ1 (which similarly to XA2 has Android 8 Oreo with a 4.4 kernel) using some inotify vs vfs_rename race condition. This exploit is kernel-specific, not device specific.

Maybe finding an old enough firmware for the Xperia XA2 (with a still unpatched kernel) could open possibilities here ?

I wonder if this would help backing up XA2 :)

"A use-after-free issue was found in the networking subsystem's sockfs code and looks like it could lead to arbitrary code execution as a result. Affects Linux kernel from early 2.6 to 4.20.11"

https://nvd.nist.gov/vuln/detail/CVE-2019-8912https://github.com/torvalds/linux/commit/9060cb719e61b685ec0102574e10337fa5f445ea

Then again quote from https://news.ycombinator.com/item?id=19210727 :

"AFAICS, this was exposed by the addition of sockfs_setattr() in v4.10. So it's incorrect to claim that kernels older than that are vulnerable, even though the code being fixed was older."

Regarding the vfs_rename race condition mentionned above : nope. not going to work.

This exploit was fixed in 2017-12 by Google. The oldest firmware for XA2 I could notice (50.1.A.4.76) is from somewhere early 2018, after the fix.

But...