[v2,08/11] meson: make the CSPRNG backend configurable

Commit Message

Patrick Steinhardt Jan. 14, 2025, 11:56 a.m. UTC

The CSPRNG backend is not configurable in Meson and isn't quite
discoverable, either. Make it configurable and add the actual backend
used to the summary.

Signed-off-by: Patrick Steinhardt <[e-mail]>
---
 meson.build       | 28 +++++++++++++++++++++-------
 meson_options.txt |  2 ++
 2 files changed, 23 insertions(+), 7 deletions(-)

Patch

diff --git a/meson.build b/meson.build
index 95baada74af8f7731867e7075d9986b32a034ff4..e50ccac5c9f6d310d8c9f83d893ea802b5827252 100644
--- a/meson.build
+++ b/meson.build
@@ -1332,6 +1332,7 @@  if not meson.is_cross_build() and fs.exists('/dev/tty')
   libgit_c_args += '-DHAVE_DEV_TTY'
 endif
 
+csprng_backend = get_option('csprng_backend')
 https_backend = get_option('https_backend')
 sha1_backend = get_option('sha1_backend')
 sha1_unsafe_backend = get_option('sha1_unsafe_backend')
@@ -1343,7 +1344,7 @@  if https_backend == 'auto' and security_framework.found()
   https_backend = 'CommonCrypto'
 endif
 
-openssl_required = 'openssl' in [https_backend, sha1_backend, sha1_unsafe_backend, sha256_backend]
+openssl_required = 'openssl' in [csprng_backend, https_backend, sha1_backend, sha1_unsafe_backend, sha256_backend]
 openssl = dependency('openssl', required: openssl_required, default_options: ['default_library=static'])
 if https_backend == 'auto' and openssl.found()
   https_backend = 'openssl'
@@ -1428,18 +1429,30 @@  else
   error('Unhandled SHA256 backend ' + sha256_backend)
 endif
 
-if compiler.has_header_symbol('stdlib.h', 'arc4random_buf')
+# 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')
   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_header_symbol('sys/random.h', 'getrandom', 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_header_symbol('unistd.h', 'getentropy', 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_header_symbol('ntsecapi.h', 'RtlGenRandom', prefix: '#include <windows.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')
@@ -1976,6 +1989,7 @@  summary({
 }, section: 'Auto-detected features')
 
 summary({
+  'csprng': csprng_backend,
   'https': https_backend,
   'sha1': sha1_backend,
   'sha1_unsafe': sha1_unsafe_backend,
diff --git a/meson_options.txt b/meson_options.txt
index 34ba679cf931b67a794a9bb7e765bfb22106381e..5429022f30621105cd6974e4260cca60e5f24324 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -47,6 +47,8 @@  option('regex', type: 'feature', value: 'auto',
   description: 'Use the system-provided regex library instead of the bundled one.')
 
 # Backends.
+option('csprng_backend', type: 'combo', value: 'auto', choices: ['auto', 'arc4random', 'arc4random_bsd', 'getrandom', 'getentropy', 'rtlgenrandom', 'openssl', 'urandom'],
+  description: 'The backend to use for generating cryptographically-secure pseudo-random numbers.')
 option('https_backend', type: 'combo', value: 'auto', choices: ['auto', 'openssl', 'CommonCrypto', 'none'],
   description: 'The HTTPS backend to use when connecting to remotes.')
 option('sha1_backend', type: 'combo', choices: ['openssl', 'block', 'sha1dc', 'CommonCrypto'], value: 'sha1dc',