mmap with file descriptor -1 in disassembled output

Question

I am reversing a binary using Ghidra. In the disassembled output, I have the following lines in the main function:

(code *)mmap((void *)0x0,0x55,7,0x22,-1,0)

I am quite confused here since the file descriptor appears to be -1 and I remember reading that file descriptors should be non-negative.

Can someone please tell me what I am missing here?

Answer 1

https://man7.org/linux/man-pages/man2/mmap.2.html

Relevant section pertaining to the flags argument (emphasis added):

MAP_ANONYMOUS: The mapping is not backed by any file; its contents are initialized to zero. The fd argument is ignored; however, some implementations require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is specified, and portable applications should ensure this. The offset argument should be zero. The use of MAP_ANONYMOUS in conjunction with MAP_SHARED is supported on Linux only since kernel 2.4.

While this seems like a likely explanation, it isn't a 100% guarantee on its own. You could find further proof by looking at the headers for the system you're REing to confirm that MAP_ANONYMOUS is indeed being used as part of the flags.