Fix undefined behaviour in variadic argument handling

[javierhonduco]

We were seeing some crashes in Parca Agent (parca-dev/parca-agent#817) when running on libbpfgo v0.4.0-libbpf-1.0.0.

fatal error: unexpected signal during runtime execution
[signal SIGSEGV: segmentation violation code=0x1 addr=0x3c pc=0x18724f1]

runtime stack:
runtime.throw({0x1da3dc1?, 0x0?})
	runtime/panic.go:992 +0x71
runtime.sigpanic()
	runtime/signal_unix.go:802 +0x3a9
[...]

Under gdb, the top of the stack seemed to be around the libbpf printing facilities:

(gdb) bt
#0  0x00000000018724f1 in __strstr_sse2_unaligned ()
#1  0x00000000017c4156 in libbpf_print_fn ()
#2  0x00000000017cb31c in libbpf_print (level=(unknown: 0x3c), level@entry=LIBBPF_WARN, format=0x205ad64 "cgroup") at libbpf.c:103
#3  0x00000000017debc7 in bpf_object__init_user_maps (obj=0x7fff9c0012a0, strict=<optimized out>) at libbpf.c:2004
#4  bpf_object__init_maps (obj=obj@entry=0x7fff9c0012a0, opts=opts@entry=0xc0000fe280) at libbpf.c:2617
#5  0x00000000017cdfe7 in bpf_object_open (path=<optimized out>, path@entry=0x0, obj_buf=0x7fff9c000b90, obj_buf_sz=1792, opts=0xc0000fe280) at libbpf.c:7410
#6  0x00000000017ce1ad in bpf_object__open_mem (obj_buf=0x205ad64, obj_buf_sz=103, opts=0x7fffa6ffbd50) at libbpf.c:7472
#7  0x00000000017c49f4 in _cgo_b2805b411fbe_C2func_bpf_object__open_mem ()
#8  0x000000000046c924 in runtime.asmcgocall () at runtime/asm_amd64.s:821
#9  0x0000000000000004 in ?? ()
#10 0x000000c00063a7d0 in ?? ()
#11 0x0000000000000001 in ?? ()
#12 0x0000000000000000 in ?? ()

Which led me to the variadic arguments being reused after iterating over them. Also added the missing va_ends before returning, as per the UNIX spec

Corresponding va_start() and va_end() macro calls must be in the same function.

https://www.ibm.com/docs/en/zos/2.3.0?topic=lf-va-arg-va-copy-va-end-va-start-access-function-arguments

Signed-off-by: Francisco Javier Honduvilla Coto javierhonduco@gmail.com

[geyslan]

As tests passed and based on the stdarg man, this LGTM. Thanks @javierhonduco

va_end()
       Each invocation of va_start() must be matched by a corresponding
       invocation of va_end() in the same function.  After the call
       va_end(ap) the variable ap is undefined.  Multiple traversals of
       the list, each bracketed by va_start() and va_end() are possible.
       va_end() may be a macro or a function.

I would only add the above reference.

--- EDIT:

Done.