Un-necessary loglinear complexity in `platform._platform`

[kexinoh]

Bug Description:
A of simple quadratic complexity code in platform.py

Vulnerability Locations :

1. cpython/Lib/platform.py

   Line 642 in 98a5b83

   while platform and platform[-1] == '-':

Repair Status:
None

Common Information:

• CPython Version: main branch
• Operating System: Linux

Linked PRs

• gh-136066: simplify platform._platform() #136069

[picnixz]

In practice, this one doesn't seem exploitable because that means that uname() must be VERY LARGE. And it's not like an adversary may change this at will. So I wouldn't consider it a security issue, just a possible improvement.

[kexinoh]

I'll modify the expression

[picnixz]

@serhiy-storchaka Historically, did we treat such cases as bugfixes, features or security fixes? I don't think it's a security issue, but it can be regarded as a small bug. Benchmarks however show no differences as platform.platform() calls are cached and since the function is private, it shouldn't be used elsewhere.

[serhiy-storchaka]

The asymptotic complexity is not quadratic, but $O(n\log{n})$. Each iteration of the first loop halves the number of repeated minuses, so there are at most $\log_2{n}$ iterations. The second loop is repeated at most once, since there is no more than one trailing minus left.

[serhiy-storchaka]

I would treat this issue as cosmetic. The code can be reduced to few lines by using re and string methods instead of manual loops.