Subject: Re: [PATCH 7/9] meson: make the CSPRNG backend configurable

From: Patrick Steinhardt <[email-masked]>
Date: Mon, 13 Jan 2025 10:25:25 +0100

On Mon, Jan 13, 2025 at 09:33:40AM +0100, Patrick Steinhardt wrote:
> diff --git a/meson.build b/meson.build
> index 5e1373f6a52a91beb527d00d8fd5c55d377c718b..cb352ce6fd50616e3281a776104692c5b2bfa5b2 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -1325,6 +1325,7 @@ if not meson.is_cross_build() and fs.exists('/dev/tty')
> @@ -1421,18 +1422,28 @@ else
>    error('Unhandled SHA256 backend ' + sha256_backend)
>  endif
>  
> -if compiler.has_header_symbol('stdlib.h', 'arc4random_buf')
> +if csprng_backend in ['auto', 'arc4random'] and compiler.has_header_symbol('stdlib.h', 'arc4random_buf', required: csprng_backend == 'arc4random')
>    libgit_c_args += '-DHAVE_ARC4RANDOM'
> -elif compiler.has_header_symbol('bsd/stdlib.h', 'arc4random_buf')
> +  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'
> -elif compiler.has_function('getrandom', prefix: '#include <sys/random.h>')
> +  csprng_backend = 'arc4random_bsd'
> +elif csprng_backend in ['auto', 'getrandom'] and compiler.has_function('getrandom', prefix: '#include <sys/random.h>', required: csprng_backend == 'getrandom')
>    libgit_c_args += '-DHAVE_GETRANDOM'
> -elif compiler.has_function('getentropy', prefix: '#include <unistd.h>')
> +  csprng_backend = 'getrandom'
> +elif csprng_backend in ['auto', 'getentropy'] and compiler.has_function('getentropy', prefix: '#include <unistd.h>', required: csprng_backend == 'getentropy')
>    libgit_c_args += '-DHAVE_GETENTROPY'
> -elif compiler.has_function('RtlGenRandom', prefix: '#include <windows.h>\n#include <ntsecapi.h>')
> +  csprng_backend = 'getentropy'
> +elif csprng_backend in ['auto', 'rtlgenrandom'] and compiler.has_function('RtlGenRandom', prefix: '#include <windows.h>\n#include <ntsecapi.h>', required: csprng_backend == 'rtlgenrandom')
>    libgit_c_args += '-DHAVE_RTLGENRANDOM'
> -elif openssl.found()
> +  csprng_backend = 'rtlgenrandom'
> +elif csprng_backend in ['auto', 'openssl'] and openssl.found()
>    libgit_c_args += '-DHAVE_OPENSSL_CSPRNG'
> +  csprng_backend = 'openssl'
> +elif csprng_backend in ['auto', 'urandom']
> +  csprng_backend = 'urandom'
> +else
> +  error('Unsupported CSPRNG backend: ' + csprng_backend)
>  endif
>  
>  if get_option('runtime_prefix')

I just noticed that this generates warnings because we use features not
yet in Meson v0.61.0, which is our minimum required version. I'll convert
these to instead use `compiler.has_header_symbol()` consistently, which
is nicer to read anyway, and will add another patch on top that makes us
use `--fatal-meson-warnings` in CI so that warnings will cause us to
abort the build.

Patrick