Fix CPU_SET dynamic allocation and leak

[prashanthswami]

The initial implementation had a number of issues:

• The allocation of the CPU_SET should be checked for a NULL return.
• The CPU_*_S macros should be used when working with dynamic sets.
• The CPU_SET needs to be cleared via CPU_ZERO_S before use.
• Dynamic CPU_SETs need to be freed after use.
• The __riscv_hwprobe syscall is expecting a set size not a count.

Reviewer: @malfet
CC: @GlassOfWhiskey @markdryan

[markdryan]

@prashanthswami I tried building cpuinfo today an a RISC-V VM (Ubuntu 23.10 with glibc 2.38) and the build failed with the following error

cpuinfo/src/riscv/linux/riscv-hw.c:1:10: fatal error: sys/hwprobe.h: No such file or directory
    1 | #include <sys/hwprobe.h>
      |          ^~~~~~~~~~~~~~~
compilation terminated.

This was the main branch without this PR, so it looks like the RISC-V builds are currently broken (they don't seem to be tested by the CI). It looks to me like you're relying on glibc's implementation of hwprobe, but AFAIK this isn't merged yet. How did you get it to build? Did you build your own glibc with the hwprobe patch applied? In any case, we can't really assume that this header will always be available, not for a good few years at least.

Other projects that are using hwprobe right now are making the syscall directly, which is probably the safest thing to do for now. OpenJDK duplicates the constants from the kernel headers needed for the sys call and then does a direct syscall. A nicer solution would be to first check to see if the <asm/hwprobe.h> exists either using a build system check or perhaps using

#ifdef __has_include
# if __has_include (<asm/hwprobe.h>)
#  include <asm/hwprobe.h>
# endif
#endif

and manually defining the constants if asm/hwprobe is not available before making the syscall.

[GlassOfWhiskey]

What I was trying to do in #148 is to check the kernel version at compile time.
If you prefer, I can do a different check, or move it directly at runtime. Please let me know what you prefer, but please do not split again the discussion over multiple PRs otherwise it becomes impossible to follow the flow.

[markdryan]

What I was trying to do in #148 is to check the kernel version at compile time. If you prefer, I can do a different check, or move it directly at runtime. Please let me know what you prefer, but please do not split again the discussion over multiple PRs otherwise it becomes impossible to follow the flow.

Where would you like to have the discussions? Over at #148 or perhaps #124?

I've answered my own question. Looks like #148 is the place to be. I'll comment there.

[prashanthswami]

+1 Let's keep discussions about swapping hwprobe includes over in #148 - this PR is purely to resolve the CPU_SET usage error.