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Message-ID: <4ab2651b-c10b-40a9-a641-d5d402ddd881@arm.com>
Date: Fri, 10 Apr 2026 18:06:44 +0200
From: Gilles Peskine <gilles.peskine@....com>
To: musl@...ts.openwall.com
Subject: Detecting getrandom() or syscall()
Hello,
I'm a maintainer of a cryptography library (TF-PSA-Crypto, recently
split out
of Mbed TLS), which is used on a wide variety of embedded systems.
Our library is mostly portable C, but we do try to detect the presence
of some
critical system features. One such feature is a source of random data. When
running on a Linux kernel, we prefer the getrandom() system call, and
fall back
to /dev/random or /dev/urandom.
Experimentally, at least with a modern musl version, we can call the
getrandom() system call with
#include <unistd.h>
#include <sys/syscall.h>
syscall(SYS_getrandom, ...) // or __NR_getrandom perhaps on some libc?
However, not all systems have <unistd.h>, <sys/syscall.h> and syscall(). So
we need to wrap this code in preprocessor directives. We know that
Linux-Glibc
and Midipix have these headers, so our code currently looks for
getrandom() on
these systems.
https://github.com/Mbed-TLS/TF-PSA-Crypto/blob/tf-psa-crypto-1.1.0/platform/platform_util.c#L297
#if ((defined(__linux__) && defined(__GLIBC__)) ||
defined(__midipix__))
#include <unistd.h>
#include <sys/syscall.h>
#if defined(SYS_getrandom)
...
syscall(SYS_getrandom, ...)
It would be nice to also use getrandom() on Linux-musl. But how can we
detect
musl? We can't just use #if defined(__linux__) since not all Linux build
environments have <sys/syscall.h> and a syscall() function.
The musl FAQ explains that there's no feature detection macro for musl
because "it's a bug to assume a certain implementation has particular
properties rather than testing". But looking at the musl headers, I can't
find anything that I could test to guarantee the presence of <sys/syscall.h>
and syscall().
--
Best regards,
Gilles Peskine
TF-PSA-Crypto and Mbed TLS maintainer
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