Context: I am trying to write a small C program with inline asm that should run under Linux on an x86_64 system and being compiled with gcc in order to better understand how syscalls work under Linux.
My question is: How are error numbers returned from a syscall (e.g. write) in this environment? I understand that when I use a library such as glibc, it takes care of saving the resulting error code in the global errno variable. But where is the error number stored when I call a syscall directly through inline assembler? Will it be stored inside a separate register, or will it be encoded in %rax?
Let take the write syscall on linux as an example:
When call write then after the syscall returns I find it stores 0xfffffffffffffff2 inside %rax, do I need to
somehow extract the error code from that?
If I have the error code number, where should I look to identify the actual error that occured? Lets say I get the number 5 returned, which header file do I need to consult to find the corresponding symbolic error name.
I am calling the write syscall like this:
asm ("mov $1,%%rax;"
"mov $1,%%rdi;"
"mov %1,%%rsi;"
"mov %2,%%rdx;"
"syscall;"
"mov %%rax,%0;"
: "=r" (result)
: "r" (msg), "r" (len)
: "%rdx", "%rsi", "%rax", "%rdi" /* EDIT: this is needed or else the registers will be overwritten */
);
with result, msg and len defined like so:
long result = 0;
char* msg = "Hello World\n";
long len = 12;
The linux syscall's convention is that they encode both the possible error code and the return value for successful call in the return value. It's just glibc or other C libraries's wrappers that they will set errno to the error code returned by the underline syscall, and the wrapper will return -1. Take the write as an example, the kernel does the error processing similar to this:
ssize_t write(int fd, ...) {
if (fd is not valid)
return -EBADF;
return do_write(...);
}
So as you can see, the error code is just in the return value, and depending on the semantics, there is always a way to check if the syscall succeeded or not by comparing it to a value not possible for successful operation. For most syscalls, like the write one, that means check if it is negative.
Architecture Calling Conventions
As you already guessed, you can't use errno because it's GLibC specific. The information you want will be in rax if it's a x86_64. The man page man 2 syscall has the following explanation:
Architecture calling conventions
Every architecture has its own way of invoking and passing arguments
to the kernel. The details for various architectures are listed in
the two tables below.
The first table lists the instruction used to transition to kernel
mode (which might not be the fastest or best way to transition to the
kernel, so you might have to refer to vdso(7)), the register used to
indicate the system call number, the register used to return the
system call result, and the register used to signal an error.
arch/ABI instruction syscall # retval error Notes
────────────────────────────────────────────────────────────────────
alpha callsys v0 a0 a3 [1]
arc trap0 r8 r0 -
arm/OABI swi NR - a1 - [2]
arm/EABI swi 0x0 r7 r0 -
arm64 svc #0 x8 x0 -
blackfin excpt 0x0 P0 R0 -
i386 int $0x80 eax eax -
ia64 break 0x100000 r15 r8 r10 [1]
m68k trap #0 d0 d0 -
microblaze brki r14,8 r12 r3 -
mips syscall v0 v0 a3 [1]
nios2 trap r2 r2 r7
parisc ble 0x100(%sr2, %r0) r20 r28 -
powerpc sc r0 r3 r0 [1]
s390 svc 0 r1 r2 - [3]
s390x svc 0 r1 r2 - [3]
superh trap #0x17 r3 r0 - [4]
sparc/32 t 0x10 g1 o0 psr/csr [1]
sparc/64 t 0x6d g1 o0 psr/csr [1]
tile swint1 R10 R00 R01 [1]
x86_64 syscall rax rax - [5]
x32 syscall rax rax - [5]
xtensa syscall a2 a2 -
And note number [5]:
[5] The x32 ABI uses the same instruction as the x86_64 ABI and
is used on the same processors. To differentiate between
them, the bit mask __X32_SYSCALL_BIT is bitwise-ORed into the
system call number for system calls under the x32 ABI. Both
system call tables are available though, so setting the bit
is not a hard requirement.
(In that man page, a table showing how to pass arguments to system calls follows. It's an interesting read.)
How are error numbers returned from a syscall (e.g. write) in this environment?:
You gotta check your rax register for the return value.
On Linux, a failed system call using the syscall assembly instruction will return the value -errno in the rax register. So in your case 0-0xfffffffffffffff2 == 0xE which is 14. So your errno is 14.
How do you find what errno 14 means? You should google search "Linux error code table" or look in errno.h and you'll find the answer.
Take a look here: http://www.virtsync.com/c-error-codes-include-errno
According to that table, 14 is EFAULT which means "Bad address".
IIRC in the x86-64 ABI, an error is transmitted from the syscall with the carry bit set. Then eax contains the errno code.
I would suggest to study the lower layers of the source code of some libc library, like musl-libc
来源：https://stackoverflow.com/questions/37167141/linux-syscalls-and-errno