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Message-ID: <dd8fc833-fcf9-44f2-9425-b6111c57f4b8@gmail.com>
Date: Sat, 11 Apr 2026 15:06:10 -0400
From: Demi Marie Obenour <demiobenour@...il.com>
To: musl@...ts.openwall.com, Gilles Peskine <gilles.peskine@....com>
Subject: Re: Detecting getrandom() or syscall()
On 4/10/26 12:06, Gilles Peskine wrote:
> 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().
One option is to assume that these headers are present on Linux.
If they are not, the build breaks and people know they need to use
a better libc.
--
Sincerely,
Demi Marie Obenour (she/her/hers)
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