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Date: Mon, 1 Jan 2018 17:03:54 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: [PATCH] Add getrandom syscall wrapper function On Mon, Jan 01, 2018 at 10:51:34PM +0100, Hauke Mehrtens wrote: > On 01/01/2018 09:47 PM, Rich Felker wrote: > > On Mon, Jan 01, 2018 at 09:31:23PM +0100, Hauke Mehrtens wrote: > >> This syscall is available since Linux 3.17 and was also implemented in > >> glibc in version 2.25. This is a pure syscall wrapper liker glibc does > >> it. > >> --- > >> include/sys/random.h | 19 +++++++++++++++++++ > >> src/linux/getrandom.c | 11 +++++++++++ > >> 2 files changed, 30 insertions(+) > >> create mode 100644 include/sys/random.h > >> create mode 100644 src/linux/getrandom.c > >> > >> diff --git a/include/sys/random.h b/include/sys/random.h > >> new file mode 100644 > >> index 00000000..5540f877 > >> --- /dev/null > >> +++ b/include/sys/random.h > >> @@ -0,0 +1,19 @@ > >> +#ifndef _SYS_RANDOM_H > >> +#define _SYS_RANDOM_H > >> +#ifdef __cplusplus > >> +extern "C" { > >> +#endif > >> + > >> +#define __NEED_size_t > >> +#define __NEED_ssize_t > >> +#include <bits/alltypes.h> > >> + > >> +#define GRND_NONBLOCK 0x0001 > >> +#define GRND_RANDOM 0x0002 > >> + > >> +ssize_t getrandom(void *buf, size_t buflen, unsigned int flags); > >> + > >> +#ifdef __cplusplus > >> +} > >> +#endif > >> +#endif > >> diff --git a/src/linux/getrandom.c b/src/linux/getrandom.c > >> new file mode 100644 > >> index 00000000..50b07df9 > >> --- /dev/null > >> +++ b/src/linux/getrandom.c > >> @@ -0,0 +1,11 @@ > >> +#include <sys/random.h> > >> +#include "syscall.h" > >> + > >> +ssize_t getrandom(void *buf, size_t buflen, unsigned int flags) > >> +{ > >> +#ifdef SYS_getrandom > >> + return syscall_cp(SYS_getrandom, buf, buflen, flags); > >> +#else > >> + return __syscall_ret(-ENOSYS); > >> +#endif > >> +} > >> -- > >> 2.11.0 > > > > The #ifdef doesn't make sense; if the definition is missing then it's > > a bug in musl source. > > Ok, If I can assume that SYS_getrandom is always defined I will remove this. > > > Aside from that I think the patch is okay but I'm not sure it's > > complete. There should probably also be getentropy(), > > Adding getentropy() should not be so hard, I can do that. > > > and we've > > discussed in the past but never reached any conclusion on whether > > there should be a fallback when the syscall doesn't exist (running on > > old kernel). > > glibc does not have a fallback for this syscall there was a long > discussion about this, see here: https://lwn.net/Articles/711013/ > As they never found a good solution for their fallback. I think musl > should also not provide a fallback. > > This is the glibc implementation: > https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=92dcaa3e2f7bf0f7f1c04cd2fb6a317df1a4e225 I think we can add it for now with no fallback to unblock things waiting on it, and also consider adding fallback. If/when we add arc4random/posix_random, we will have a perfectly reasonable backend we can use in the absence of the syscall. It would initialize its entropy from AT_RANDOM on kernels that have it and from the legacy sysctl random source on kernels too old to have AT_RANDOM. Rich
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