Hi, On Sun, Jun 23, 2019 at 09:03:31AM -0700, Naveen Iyer wrote: > Hi, > > As suggested by > https://www.kernel.org/doc/man-pages/reporting_bugs.html , I looked up > the online man page as well: > http://man7.org/linux/man-pages/man2/syscall.2.html > > In the table in "Architecture calling conventions" section: > arch/ABI instruction syscall # retval error Notes > ──────────────────────────────────── > x86-64 syscall rax rax - [5] > > Retval must be placed in rdi Why? > as per the following link: > https://w3challs.com/syscalls/?arch=x86_64 This link doesn't claim that. > I ran the test to verify that the retval is indeed supposed to be > placed in rdi, and not rax as the man page incorrectly suggests. > My man page version: > $ man --version > man 2.8.3 > > Steps to replicate this issue: > Step 1) > Write test code 'startup.c': > void _start() > { > /* > Source: $: man syscall > arch/ABI instruction syscall # retval error Notes > ──────────────────────────────────────────────────────────────────── > i386 int $0x80 eax eax - > ia64 break 0x100000 r15 r8 r10 [1] > x86-64 syscall rax rax - [5] > */ > > // Specify the return value/error code in ebx for x86 and rdi for x86_64 > asm volatile ( > "movq $97, %rdi" > ); %rdi contains the first argument of syscall on x86_64. > // Specify the type of system call (1 is the exit syscall) in > eax register. > asm volatile ( > "movq $0x3c, %rax" > ); syscall number 0x3c is exit on x86_64. > > // Transition to kernel mode. > asm volatile ( > "syscall" > ); This syscall does not return. > } > > Step 2) Compile and execute: > $ gcc -nostdlib -c startup.c > $ ld startup.o -o startup > $ ./startup > $ echo $? > 97 So your test invokes "exit" syscall with 97 as its first argument. The return value of this syscall has no meaning because it does not return. Most of syscalls do return and their return value is in %rax on x86_64. -- ldv
