So, I’ve thought I’d implement a brk() system call really quickly in
my tiny libc library, but I’ve run into complications. Why? Short
answer: I read the documentation wrong. sbrk() returns the old
address, not the new one!
But anyways, I’ve learned some other interesting things along the way.
For odd reasons that be, I was testing on Ubuntu 10.04 LTS, which has
this weird bug that under certain conditions like programs being
spawned from make, memory maps are setup such that brk() is stuck
between two other allocations and can't move. Thus, only mmap() can
be used on this particular Linux configuration to get more memory.
That's dumb. mmap()` is too complicated to be recommendable for use
in very small, simple, statically-linked Linux programs.
Luckily, however, I was able to get sbrk() working even on this
system.
Now it turns out there’s an easy way to check a process’s memory maps on Linux:
cat /proc/$PID/maps
20180615/DuckDuckGo linux kernel sbrk
20180615/https://elinux.org/images/b/b5/Elc2013_Kobayashi.pdf
20180615/DuckDuckGo ubuntu sbrk fail
20180615/https://bugs.chromium.org/p/chromium/issues/detail?id=57908
20180615/https://unix.stackexchange.com/questions/329111/enomem-when-calling-sbrk-for-8gb
20180615/DuckDuckGo linux sbrk security limit object
20180615/https://stackoverflow.com/questions/997425/how-are-sbrk-brk-implemented-in-linux
20180615/DuckDuckGo linux sbrk always fails
20180615/https://stackoverflow.com/questions/6988487/what-does-the-brk-system-call-do
20180615/http://www.howtobuildsoftware.com/index.php/how-do/NM3/linux-malloc-system-calls-dynamic-memory-allocation-sbrk-why-my-implementation-of-sbrk-system-call-does-not-work
Also, if you’re wondering about the special system call instruction
for 64-bit Linux. It turns out that there is also a variant that only
works on 32-bit systems, sysenter/sysexit that is used by Windows
XP. syscall/sysret is what is used on modern 64-bit systems, but
it only works on pure 64-bit systems, no compatibility modes
enabled. Also, sysret has a silicon bug that causes a security
vulnerability on all known modern architectures. Ah, that’s a real
bummer.
20180615/DuckDuckGo 64 bit linux system instruction
20180615/https://stackoverflow.com/questions/8510333/x86-64-assembly-linux-system-call-confusion
20180615/https://wiki.osdev.org/SYSENTER