On Sun, Mar 16, 2025 at 01:41:40PM -0700, Junio C Hamano wrote: > "brian m. carlson" <sandals@xxxxxxxxxxxxxxxxxxxx> writes: > > > When arc4random was added to glibc, the Linux kernel CSPRNG maintainer > > argued that it was not a secure approach (I disagree), and convinced the > > glibc maintainers to just make it a wrapper around the Linux kernel > > CSPRNG, which it now is. So there's no actual benefit to calling > > arc4random versus getrandom, and since it's newer and less commonly > > available than getrandom, as well as slightly slower (because of an > > extra function call), getrandom should be preferred. > > This > > https://www.phoronix.com/news/GNU-Glibc-arc4random-Functions > > was the first hit of my search in the area, but I think you are > referring to > > https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=eaad4f9 > > that happened 5 days after the thing got in and the code there tells > me that your summary of the situation is quite accurate. > > So I agree that dropping this patch makes sense, but do we want to > do a bit more to improve the situation? > > Here is an attempt to improve what we have in Makefile (and possibly > the Linux section in config.mak.uname, but that is improving what we > do not have) to tell folks that arc4random in glibc is only for > compatibility and they should pick getrandom() until the situation > changes. > > --- >8 --- > Subject: config/Makefile: a note on CSPRNG_METHOD choice for Linux > > arc4random() was added to glibc in July 2022, but quickly replaced > by a stub implementation that wraps around getrandom(). Hence there > is no actual benefit to calling arc4random() over getrandom() on > glibc based systems, at least for now. > > To avoid enticing Linux users to choose arc4random(), leave a note > that their arc4random() in glibc is not the same as what their > friends use on other platforms, and guide them to use getrandom() > instead in the meantime. Makes me wonder whether we should also change the order in which Meson auto-detects functions. That is, do we want the following patch that favors getrandom over arc4random? Patrick diff --git a/meson.build b/meson.build index d6e27b236fa..501b2becabb 100644 --- a/meson.build +++ b/meson.build @@ -1481,15 +1481,15 @@ endif # Backends are ordered to reflect our preference for more secure and faster # ones over the ones that are less so. -if csprng_backend in ['auto', 'arc4random'] and compiler.has_header_symbol('stdlib.h', 'arc4random_buf', required: csprng_backend == 'arc4random') +if csprng_backend in ['auto', 'getrandom'] and compiler.has_header_symbol('sys/random.h', 'getrandom', required: csprng_backend == 'getrandom') + libgit_c_args += '-DHAVE_GETRANDOM' + csprng_backend = 'getrandom' +elif csprng_backend in ['auto', 'arc4random'] and compiler.has_header_symbol('stdlib.h', 'arc4random_buf', required: csprng_backend == 'arc4random') libgit_c_args += '-DHAVE_ARC4RANDOM' csprng_backend = 'arc4random' elif csprng_backend in ['auto', 'arc4random_bsd'] and compiler.has_header_symbol('bsd/stdlib.h', 'arc4random_buf', required: csprng_backend == 'arc4random_bsd') libgit_c_args += '-DHAVE_ARC4RANDOM_BSD' csprng_backend = 'arc4random_bsd' -elif csprng_backend in ['auto', 'getrandom'] and compiler.has_header_symbol('sys/random.h', 'getrandom', required: csprng_backend == 'getrandom') - libgit_c_args += '-DHAVE_GETRANDOM' - csprng_backend = 'getrandom' elif csprng_backend in ['auto', 'getentropy'] and compiler.has_header_symbol('unistd.h', 'getentropy', required: csprng_backend == 'getentropy') libgit_c_args += '-DHAVE_GETENTROPY' csprng_backend = 'getentropy'
