config.mak.uname: set CSPRNG_METHOD to getrandom on Linux

Commit 05cd988dce ("wrapper: add a helper to generate numbers from a
CSPRNG", 2022-01-17) added a csprng_bytes() function which used one
of several interfaces to provide a source of cryptographically secure
pseudorandom numbers. The CSPRNG_METHOD make variable was provided to
determine the choice of available 'backends' for the source of random
bytes.

Commit 05cd988dce did not set CSPRNG_METHOD in the Linux section of
the config.mak.uname file, so it defaults to using '/dev/urandom' as
the source of random bytes. The 'backend' values which could be used
on Linux are 'arc4random', 'getrandom' or 'getentropy' ('openssl' is
an option, but seems to be discouraged).

The arc4random routines (arc4random_buf() is the one actually used) were
added to glibc in version 2.36, while both getrandom() and getentropy()
were included in 2.25. So, some of the more up-to-date distributions of
Linux (eg Debian 12, Ubuntu 24.04) would be able to use the 'arc4random'
setting. All currently supported distributions have glibc 2.25 or later
(RHEL 8 has v2.28) and, therefore, have support for the 'getrandom' and
'getentropy' settings.

The arc4random routines on the *BSDs (along with cygwin) implement the
ChaCha20 stream cipher algorithm (see RFC8439) in userspace, rather than
as a system call, and are thus somewhat faster (having avoided a context
switch to the kernel). In contrast, on Linux all three functions are
simple wrappers around the same kernel CSPRNG syscall.

If the meson build system is used on a newer platform, then they will be
configured to use 'arc4random', whereas the make build will currently
default to using '/dev/urandom' on Linux. Since there is no advantage,
in terms of performance, to the 'arc4random' setting, the 'getrandom'
setting should be preferred from an availability perspective. (Also, the
current uses of csprng_bytes() are not in any hot path).

In order to set an appropriate default, set the CSPRNG_METHOD build
variable to 'getrandom' in the Linux section of the 'config.mak.uname'
file.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.uname

@@ -50,6 +50,7 @@ ifeq ($(uname_S),Linux)
 	HAVE_ALLOCA_H = YesPlease
 	# override in config.mak if you have glibc >= 2.38
 	NO_STRLCPY = YesPlease
+	CSPRNG_METHOD = getrandom
 	HAVE_PATHS_H = YesPlease
 	LIBC_CONTAINS_LIBINTL = YesPlease
 	HAVE_DEV_TTY = YesPlease