Fix #19052; [backport:1.6.0] (#19053)

* Fix #19052; [backport:1.6.0]

Adds a compile flag to avoid a getrandom syscall, fixing #19052.

This is neccesary when the getrandom syscall is missing, as noted in #19052, particularly in kernel versions < 3.17 when getrandom was introduced. Specifically relevant is this is missing from kernel 3.10, which is the supported kernel throughout RHEL 7 and CentOS 7, which is widely used at many organizations. Without this, versions of nim that include sysrand (i.e. versions >= 1.6.0) will not compile without modification, however with this change a compile flag may be used to fall back using /dev/urandom as done with any unknown Posix OS (preferred here as a fallback since it already supplies a cryptographically secure PRNG and existing code deals with entropy pool init, etc).

The change is placed behind a compile flag, as discussed in github ticket #19052 (summed up here):
* First, I can't seem to catch that a importc such as SYS_getrandom is declared without using it (the declared proc returns true, but compiler throws an undeclared identifier flag when referencing it).
* Second, it seemed preferable to be behaviorally explicit vs implicit when considering this is intended to be a cryptographically secure PRNG.
* Third, if I intend to compile on a kernel >= 3.17 while running the binary on at least one system < 3.17, I'll want to be able to target this without relying on a compile time determination if the getrandom syscall is available.

* Documenting compile flag for -d:nimNoGetRandom and adding changelog entry
Related to #19052 and comments in PR #19053. Also created a new changelog file since none currently exists.

Co-authored-by: Timothy Alexander <talexander@midwestlabs.com>

2 files changed, 37 insertions, 1 deletions

diff --git a/changelogs/changelog.md b/changelogs/changelog.md
new file mode 100644
index 000000000..475f568ec
--- /dev/null
+++ b/changelogs/changelog.md
@@ -0,0 +1,31 @@
+# v1.xx.x - yyyy-mm-dd
+
+## Changes affecting backward compatibility
+
+## Standard library additions and changes
+
+### New compile flag (`-d:nimNoGetRandom`) when building `std/sysrand` to remove dependency on linux `getrandom` syscall
+
+This compile flag only affects linux builds and is necessary if either compiling on a linux kernel version < 3.17, or if code built will be executing on kernel < 3.17.
+
+On linux kernels < 3.17 (such as kernel 3.10 in RHEL7 and CentOS7), the `getrandom` syscall was not yet introduced. Without this, the `std/sysrand` module will not build properly, and if code is built on a kernel >= 3.17 without the flag, any usage of the `std/sysrand` module will fail to execute on a kernel < 3.17 (since it attempts to perform a syscall to `getrandom`, which isn't present in the current kernel). A compile flag has been added to force the `std/sysrand` module to use /dev/urandom (available since linux kernel 1.3.30), rather than the `getrandom` syscall. This allows for use of a cryptographically secure PRNG, regardless of kernel support for the `getrandom` syscall.
+
+When building for RHEL7/CentOS7 for example, the entire build process for nim from a source package would then be:
+```sh
+$ yum install devtoolset-8 # Install GCC version 8 vs the standard 4.8.5 on RHEL7/CentOS7. Alternatively use -d:nimEmulateOverflowChecks. See issue #13692 for details
+$ scl enable devtoolset-8 bash # Run bash shell with default toolchain of gcc 8
+$ sh build.sh  # per unix install instructions
+$ bin/nim c koch  # per unix install instructions
+$ ./koch boot -d:release  # per unix install instructions
+$ ./koch tools -d:nimNoGetRandom  # pass the nimNoGetRandom flag to compile std/sysrand without support for getrandom syscall
+```
+
+This is necessary to pass when building nim on kernel versions < 3.17 in particular to avoid an error of "SYS_getrandom undeclared" during the build process for stdlib (sysrand in particular).
+
+## Language changes
+
+
+## Compiler changes
+
+
+## Tool changes
diff --git a/lib/std/sysrand.nim b/lib/std/sysrand.nim
index 75983e3c6..b35f24a72 100644
--- a/lib/std/sysrand.nim
+++ b/lib/std/sysrand.nim
@@ -38,6 +38,11 @@
 ## .. _randomFillSync: https://nodejs.org/api/crypto.html#crypto_crypto_randomfillsync_buffer_offset_size
 ## .. _/dev/urandom: https://en.wikipedia.org/wiki//dev/random
 ##
+## On a Linux target, a call to the `getrandom` syscall can be avoided (e.g.
+## for targets running kernel version < 3.17) by passing a compile flag of
+## `-d:nimNoGetRandom`. If this flag is passed, sysrand will use `/dev/urandom`
+## as with any other POSIX compliant OS.
+##
 
 runnableExamples:
   doAssert urandom(0).len == 0
@@ -159,7 +164,7 @@ elif defined(windows):
 
     result = randomBytes(addr dest[0], size)
 
-elif defined(linux):
+elif defined(linux) and not defined(nimNoGetRandom):
   # TODO using let, pending bootstrap >= 1.4.0
   var SYS_getrandom {.importc: "SYS_getrandom", header: "<sys/syscall.h>".}: clong
   const syscallHeader = """#include <unistd.h>