utils: unpack kheaders.tar.xz if necessary

[mokomull]

When CONFIG_IKHEADERS is enabled, a tarball containing the kernel
headers is available in /sys/kernel, allowing the kernel type
definitions to be used even if the headers have not been separately
distributed to /lib on the target.

This is akin to how bcc handles it, but with the new path in /sys
after 2. Notably, use the same path in /tmp, so that the tarball will
be unpacked only once between the two tools.

[joelagnel]

Thanks for doing this. Could you read the /tmp directory path from $TMPDIR if it exists? If it doesn't then default to /tmp.

[danobi]

Sounds reasonable.

[mokomull]

I added the functionality to use getenv("TMPDIR") if present; it seems to work as expected:

Without $TMPDIR set, this does:

2692  execve("./src/bpftrace", ["./src/bpftrace", "--include", "linux/blkdev.h", "-e", "tracepoint:nbd:nbd_send_request "...], 0x7ffed5225e68 /* 12 vars */) = 0
2693  execve("/bin/sh", ["sh", "-c", "tar xf /sys/kernel/kheaders.tar."...], 0x7ffd38230288 /* 12 vars */) = 0
2693  execve("/usr/bin/tar", ["tar", "xf", "/sys/kernel/kheaders.tar.xz", "-C", "/tmp/kheaders-nyIVgE"], 0x55e096604c50 /* 15 vars */) = 0
2694  execve("/usr/local/sbin/xz", ["xz", "-d"], 0x7fffd762f4b8 /* 15 vars */) = -1 ENOENT (No such file or directory)
2694  execve("/usr/local/bin/xz", ["xz", "-d"], 0x7fffd762f4b8 /* 15 vars */) = -1 ENOENT (No such file or directory)
2694  execve("/usr/bin/xz", ["xz", "-d"], 0x7fffd762f4b8 /* 15 vars */) = 0
[...]
2692  rename("/tmp/kheaders-nyIVgE", "/tmp/kheaders-5.3.0-rc2+") = 0

and with $TMPDIR set:

2700  execve("./src/bpftrace", ["./src/bpftrace", "--include", "linux/blkdev.h", "-e", "tracepoint:nbd:nbd_send_request "...], 0x7ffeee7b8038 /* 13 vars */) = 0
2701  execve("/bin/sh", ["sh", "-c", "tar xf /sys/kernel/kheaders.tar."...], 0x7ffc903ce4e8 /* 13 vars */) = 0
2701  execve("/usr/bin/tar", ["tar", "xf", "/sys/kernel/kheaders.tar.xz", "-C", "/tmp/hi/kheaders-Kj6Lu4"], 0x56401da74630 /* 16 vars */) = 0
2702  execve("/usr/local/sbin/xz", ["xz", "-d"], 0x7ffec6e757a8 /* 16 vars */) = -1 ENOENT (No such file or directory)
2702  execve("/usr/local/bin/xz", ["xz", "-d"], 0x7ffec6e757a8 /* 16 vars */) = -1 ENOENT (No such file or directory)
2702  execve("/usr/bin/xz", ["xz", "-d"], 0x7ffec6e757a8 /* 16 vars */) = 0
[...]
2700  rename("/tmp/hi/kheaders-Kj6Lu4", "/tmp/hi/kheaders-5.3.0-rc2+") = 0

The behavior when $TMPDIR is set, but is not a directory, could possibly be improved; I'm open to feedback on this:

[mmullins@arch build]$ touch /tmp/bye
[mmullins@arch build]$ sudo TMPDIR=/tmp/bye ./src/bpftrace --include linux/blkdev.h -e 'tracepoint:nbd:nbd_send_request { printf("%d", args->request->cmd_flags); }'
terminate called after throwing an instance of 'std::runtime_error'
  what():  creating temporary path for kheaders.tar.xz failed
Aborted

This codepath is only hit if none of /lib/modules/$(uname -r)/{source,build} are present, though, so there's no regression for currently-working scenarios that I can imagine.

[danobi]

Mostly LGTM, couple nits

[danobi]

Suggested change

	FILE* tar = popen(("tar xf /sys/kernel/kheaders.tar.xz -C " + tmpdir.path).c_str(), "w");
	FILE* tar = ::popen(("tar xf /sys/kernel/kheaders.tar.xz -C " + tmpdir.path).c_str(), "w");

[danobi]

Suggested change

	int rc = pclose(tar);
	int rc = ::pclose(tar);

[danobi]

Any reason not to instead make everything public?

[mokomull]

Yeah, I wondered that one myself when I went back to post v2 yesterday :) That should shave off a few lines of weirdness.

[danobi]

LGTM, thanks!