Hacker News Reader

Comment by marbu | Hacker News Reader

Maybe because the author prefers to play with kernel. Other option is create a wrapper library and enforce it's usage with LD_PRELOAD.

View on HN

Using LD_PRELOAD you cannot hook a syscall like open().

How does strace hook it?

With a system call, ptrace(2).

ptrace.

Doesn't pretty much everything call into libc to open() things rather than invoke the syscall directly?

With LD_PRELOAD you can hook any _function_ call. A syscall is something different. It depends how the syscall wrapper within the libc is coded. Some can be hooked...

But open(3) is a function call, so it can be hooked.

It's open(2). Yes, here on my x86_64 it can be hooked. But I've seen lots of setups where this was not doable. As I said it depends how the libc's wrapper is written.

On Linux, there's typically both an open(3) and an open(2). The former is implemented by glibc and calls the latter which is implemented by the kernel.

So an app calls the open() function. This results in an unresolved symbol which the run-time linker matches up with the open() exported by glibc. glibc then does the open() syscall.

open(2) is the wrapper within the libc. There is no open(3) on Linux.

Section 2 of the unix manual is for syscalls. Section 3 is for C library functions. http://en.wikipedia.org/wiki/Man_page#Manual_sections

If libc implements it, then it is by definition in section 3.

EDIT: (signing out of this sub-thread)

Sorry, you're just wrong.

Have you ever called a system call by hand using assembly?

Have you ever looked at glibc's source?

Habe you ever looked at the kernel's close() syscall?

syscall number 5 you mean?

libc implements a function called open. Its documentation appears in section 3 of the UNIX manual. Because of that, we call it open(3). Linux implements a system call called open. Its documentation appears in section 2 of the UNIX manual. Because of that, we call it open(2). From assembly, you manually call open(2). From a C program linked against libc, you typically open(3). open(3) can be hooked, because it's just a regular function. open(2) is not a function, and so cannot be hooked like a function.

No. Assembly code invokes kernel syscalls by invoking an interrupt or using a syscall opcode. No libc or function calls involved, so LD_PRELOAD can't hook in.

You might even have several libc's in your system. You might have a uClibc + Busybox based initrd and a full glibc based root system.

Btw. how does LD_PRELOAD act together static binaries?

I know apps can invoke kernel syscalls directly, and I'm sure you could name a handful of apps that do this all time. But are you really doing that in your mp3 player?

Since static binaries don't use shared libraries, LD_PRELOAD has no effect - it is the same as with assembly code you mentioned.

Reply on news.ycombinator.com