mprotect system call not setting the errno if it fails

[shaikshavali1]

Original Test case: The original test case performs below operation. sub-test1: Invoke mprotect() with an address of 0. Check if error is set to ENOMEM. sub-test2: Invoke mprotect() with an address that is not a multiple of PAGESIZE. EINVAL sub-test3: Mmap a file with only read permission (PROT_READ). Try to set write permission (PROT_WRITE) using mprotect(2). Check that error is set to EACCES.

Link: https://github.com/lsds/ltp/blob/8b47ecbf8c1d954343316245b55e232077abcf9f/testcases/kernel/syscalls/mprotect/mprotect01.c#L83

About mprotect: On success, mprotect() and pkey_mprotect() return zero. On error, these system calls return -1, and errno is set appropriately.

Problem/issue: errno is not set while returning from system call. ocall should also return errno from host. This test invokes mprotect library API which eventually ends up in calling the host side mprotect.

Library API Link: https://github.com/lsds/sgx-lkl-musl/blob/26b0e055074230d0a90aebfefd03836088c7bb66/src/mman/mprotect.c#L5

Host Side Code Link: https://github.com/lsds/sgx-lkl/blob/5a0b5d1a31e1cbd73dc0acfe1058c7ed5d0643b3/src/host_interface/host_interface_calls.c#L92

When it returns from ocall it will pass only the return value, not the error code. After returning from the ocall, error number in the enclave is not set.

Link to ocall: https://github.com/lsds/sgx-lkl/blob/5a0b5d1a31e1cbd73dc0acfe1058c7ed5d0643b3/src/lkl/syscall-overrides-mem.c#L138

Link to setting errno: https://github.com/lsds/sgx-lkl-musl/blob/26b0e055074230d0a90aebfefd03836088c7bb66/src/internal/syscall_ret.c#L6

Complete GDB Logs:

[[  SGX-LKL ]] libc_start_main_stage2(): Calling app main: /ltp/testcases/kernel/syscalls/mprotect/mprotect01
[Switching to Thread 0x7fff91a7b700 (LWP 5791)]

Thread 6 "ENCLAVE" hit Breakpoint 1, main (ac=1, av=0x7fe040afef78) at mprotect01.c:84
84      mprotect01.c: No such file or directory.
(gdb) b mprotect
Breakpoint 2 at 0x7fe00055d46c: mprotect. (2 locations)
(gdb) c
Continuing.
Thread 6 "ENCLAVE" hit Breakpoint 2, __mprotect (addr=0x0, len=4097, prot=1) at src/mman/mprotect.c:8
8               start = (size_t)addr & -PAGE_SIZE;
(gdb) list
3       #include "syscall.h"
4
5       int __mprotect(void *addr, size_t len, int prot)
6       {
7               size_t start, end;
8               start = (size_t)addr & -PAGE_SIZE;
9               end = (size_t)((char *)addr + len + PAGE_SIZE-1) & -PAGE_SIZE;
10              return syscall(SYS_mprotect, start, end-start, prot);
10              return syscall(SYS_mprotect, start, end-start, prot);
11      }
12
(gdb) b mprotect.c:11
Breakpoint 3 at 0x7fe00055d4c6: file src/mman/mprotect.c, line 11.
(gdb) b lkl_sys
lkl_sys_access                  lkl_sys_fstat                   lkl_sys_mmap2                   lkl_sys_umount
lkl_sys_chdir                   lkl_sys_fstatat                 lkl_sys_mount                   lkl_sys_unlink
lkl_sys_chmod                   lkl_sys_getpid                  lkl_sys_newfstatat              lkl_sys_unlinkat
lkl_sys_chroot                  lkl_sys_halt                    lkl_sys_open                    lkl_sys_virtio_mmio_device_add
lkl_sys_clock_settime           lkl_sys_mkdir                   lkl_sys_openat                  lkl_syscall
lkl_sys_close                   lkl_sys_mkdirat                 lkl_sys_read                    lkl_sysctl
lkl_sys_faccessat               lkl_sys_mknod                   lkl_sys_rmdir                   lkl_sysctl_parse_write
lkl_sys_fchmodat                lkl_sys_mknodat                 lkl_sys_stat
(gdb) b lkl_sysc
lkl_syscall             lkl_sysctl              lkl_sysctl_parse_write
(gdb) b lkl_syscall
Breakpoint 4 at 0x7fe000088df0: file arch/lkl/kernel/syscalls.c, line 132.
(gdb) b run_
run_cmd                 run_init_process        run_irq                 run_ksoftirqd           run_pages_job           run_timer_softirq
run_complete_job        run_io_job              run_irqs                run_local_timers        run_posix_cpu_timers    run_timer_softirq.part
(gdb) b run_
run_cmd                 run_init_process        run_irq                 run_ksoftirqd           run_pages_job           run_timer_softirq
run_complete_job        run_io_job              run_irqs                run_local_timers        run_posix_cpu_timers    run_timer_softirq.part
(gdb) b syscalls.c:47
Breakpoint 5 at 0x7fe000088da0: file arch/lkl/kernel/syscalls.c, line 47.
(gdb) b syscalls.c:273
Breakpoint 6 at 0x7fe000088f0b: file arch/lkl/kernel/syscalls.c, line 273.
(gdb) b sgxlkl_host_syscall_mprotect
Breakpoint 7 at 0x40016a34: sgxlkl_host_syscall_mprotect. (2 locations)
(gdb) c
Continuing.

Thread 6 "ENCLAVE" hit Breakpoint 4, lkl_syscall (no=226, params=0x7fe040afee20) at arch/lkl/kernel/syscalls.c:132
132     {
(gdb) c
Continuing.

Thread 6 "ENCLAVE" hit Breakpoint 7, sgxlkl_host_syscall_mprotect (_retval=0x7fe040afed98, addr=0x0, len=8192, prot=1) at enclave/sgxlkl_t.c:776
776         oe_result_t _result = OE_FAILURE;
(gdb) c
Continuing.
Thread 6 "ENCLAVE" hit Breakpoint 7, sgxlkl_host_syscall_mprotect (addr=0x0, len=8192, prot=1) at host_interface/host_interface_calls.c:94
94          return mprotect(addr, len, prot);
(gdb) c
Continuing.

Thread 6 "ENCLAVE" hit Breakpoint 2, mprotect () at ../sysdeps/unix/syscall-template.S:78
78      ../sysdeps/unix/syscall-template.S: No such file or directory.
(gdb) c
Continuing.

Thread 6 "ENCLAVE" hit Breakpoint 6, lkl_syscall (no=226, params=0x0) at arch/lkl/kernel/syscalls.c:274
274     }
(gdb) p ret
$1 = 4294967295
(gdb) p errno
$2 = 0
(gdb) s
__filter_syscall3 (n=226, a1=0, a2=8192, a3=1) at ./src/internal/syscall.h:83
83              return res;
(gdb) p res
$3 = 4294967295
(gdb) p errno
$4 = 0
(gdb) s
84      }
(gdb) s
__syscall_ret (r=4294967295) at src/internal/syscall_ret.c:6
6               if (r > -4096UL) {
(gdb) p r
$5 = 4294967295
(gdb) p -4096UL
$6 = 18446744073709547520
(gdb) s
10              return r;
(gdb) s
11      }
(gdb) s