Issue 27278: py_getrandom() uses an int for syscall() result

syscall() result type is long.

Moreover, long type might can smaller than the size type, so we may need to add:

n = Py_MIN(size, LONG_MAX);

According to <http://man7.org/linux/man-pages/man2/getrandom.2.html>, getrandom() returns no more than 32 MiB as an int on Linux. Doesn’t that mean you can rely on syscall()’s long return value fitting in an int? Maybe just cast n = (int)sycall(...) to be explicit.

But it does make sense to cap n to LONG_MAX just in case there is some strange platform where it matters :)

Make that INT_MAX. Or change n from an int to a Py_ssize_t. Both Linux and Solaris versions or getrandom() are documented as accepting size_t buflen.

New changeset e028e86a5b73 by Victor Stinner in branch '3.5':
Fix os.urandom() using getrandom() on Linux
https://hg.python.org/cpython/rev/e028e86a5b73

New changeset 0d39bd9028e8 by Victor Stinner in branch 'default':
Merge 3.5 (os.urandom, issue #27278)
https://hg.python.org/cpython/rev/0d39bd9028e8

Martin: What do you think of my change? Is it enough? Or would you prefer an explicit cast on syscall() result?

I hesitated to use a wider type since the manual page shows an "int" type, not long.

Yeah I think your change is enough.

Adding a cast would solve this compiler warning:

Python/random.c:168:17: warning: conversion to ‘int’ from ‘long int’ may alter its value [-Wconversion]
             n = syscall(SYS_getrandom, dest, n, flags);
                 ^

But on the other hand, when I just recompiled Python with -Wconversion, I got hundreds of other warnings, so maybe there is a good reason we don’t enable that warning.

New changeset 193f50babfa4 by Victor Stinner in branch '3.5':
py_getrandom(): use long type for the syscall() result
https://hg.python.org/cpython/rev/193f50babfa4

> Adding a cast would solve this compiler warning:

I changed the code to use the long type. It should fix the warning, even if it was no more a real bug: the original bug was already fixed.

I close the issue, it's now solved.