I don't get why we're translating from kernel to userspace scores here if we just callSetpriority() with this value. Shouldn't we need to translate back?

Thegetpriorityman page says the following:

The getpriority system call returns nice values translated to the
range 40..1, since a negative return value would be interpreted
as an error. The glibc wrapper function for getpriority()
translates the value back according to the formula
unice = 20 - knice (thus, the 40..1 range returned by the kernel
corresponds to the range -20..19 as seen by user space).

Further up it says the following forsetpriority:

The prio argument is a value in the range -20 to 19 (but see
NOTES below), with -20 being the highest priority and 19 being
the lowest priority. Attempts to set a priority outside this
range are silently clamped to the range. The default priority is
0; lower values give a process a higher scheduling priority.

Pretty confusing tbh

Woof. OK, initially read this as glibc doing the translation in both cases, but I guess it's only the get operation that has issues with negative numbers.