sysctl.h deprecated in glibc 2.30

[fosslinux]

In glibc 2.30, <sysctl/sysctl.h> is deprecated, and this is issues as a warning when it is included. Because -Werror=cpp is enabled, this results in an error.

Likely the best cause of action is to remove sysctl, and replace it with reading from /proc, as mentioned in the release notes.

* The Linux-specific <sys/sysctl.h> header and the sysctl function have been
   deprecated and will be removed from a future version of glibc.
   Application should directly access /proc instead.  For obtaining random
   bits, the getentropy function can be used.

Relevant error:

In file included from crypto/random/seed/LinuxRandomUuidSysctlRandomSeed.c:23:0:
/usr/include/sys/sysctl.h:21:2: error: #warning "The <sys/sysctl.h> header is deprecated and will be removed." [-Werror=cpp]
 #warning "The <sys/sysctl.h> header is deprecated and will be removed."
  ^
cc1: all warnings being treated as errors

Other relevant issues: #20

[fosslinux]

Upon further inspection, it seems that the only Linux place where sysctl is used in crypto/random/seed/LinuxRandomUuidSysctlRandomSeed.c. So, as from the release notes: For obtaining random bits, the getentropy function can be used.

I currently see two options:

1) Remove sysctl entirely for getting random on linux

2) Disable sysctl for getting random bits when there is glibc involved.

Thoughts?

[cjdelisle]

So we could just drop the SysctlRandomSeed entirely, we have other seeding functions which use everything from /proc to __SYS_getrandom. The reason this has lived for so long is because of the chance that there might be a system where /proc actually doesn't work (suppose it has a file descriptor limit of zero). If it knows that it didn't get a random seed, it will abort, but if for some reason it thinks it got random data but in fact it got constant data (suppose someone was evil and linked /dev/urandom to /dev/zero), it would generate keys which are trivially cracked. That said, sysctl() has been deprecated for years now and I suppose it is not unreasonable to sunset it at this point.

[sdgathman]

@fosslinux

LinuxRandomUuidSysctlRandomSeed actually calls SYS_getrandom - with a fallback to sysctl when getrandom is not available. This pull request (which is merged in crashey) ifdefs away sysctl when SYS_getrandom seems to be available. https://github.com/cjdelisle/cjdns/pull/1200/commits/bc524d74a3ebfb5dafd88fef078ebf87cab7b8df

That stops the compiler warnings/errors.

[fosslinux]

ah! I did notice the use of getrandom. That does fix this specific issue; I think I'll keep this issue open for now just so @cjdelisle's idea is noticed.

[cjdelisle]

fine with me to close this because there is a solution in crashey which will eventually get merged

[fosslinux]

great