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SubjectRe: pt_regs->ax == -ENOSYS
On 4/27/21 2:28 PM, Andy Lutomirski wrote:
>> On Apr 27, 2021, at 2:15 PM, H. Peter Anvin <> wrote:
>> Trying to stomp out some possible cargo cult programming?
>> In the process of going through the various entry code paths, I have to admit to being a bit confused why pt_regs->ax is set to -ENOSYS very early in the system call path.
> It has to get set to _something_, and copying orig_ax seems perhaps silly. There could also be code that relies on ptrace poking -1 into the nr resulting in -ENOSYS.

Yeah. I obviously ran into this working on the common entry-exit code
for FRED; the frame has annoyingly different formats because of this,
and I wanted to avoid slowing down the system call path.

>> What is perhaps even more confusing is:
>> __visible noinstr void do_syscall_64(struct pt_regs *regs, unsigned long nr)
>> {
>> nr = syscall_enter_from_user_mode(regs, nr);
>> instrumentation_begin();
>> if (likely(nr < NR_syscalls)) {
>> nr = array_index_nospec(nr, NR_syscalls);
>> regs->ax = sys_call_table[nr](regs);
>> #ifdef CONFIG_X86_X32_ABI
>> } else if (likely((nr & __X32_SYSCALL_BIT) &&
>> (nr & ~__X32_SYSCALL_BIT) < X32_NR_syscalls)) {
>> nr = array_index_nospec(nr & ~__X32_SYSCALL_BIT,
>> X32_NR_syscalls);
>> regs->ax = x32_sys_call_table[nr](regs);
>> #endif
>> }
>> instrumentation_end();
>> syscall_exit_to_user_mode(regs);
>> }
>> #endif
>> Now, unless I'm completely out to sea, it seems to me that if syscall_enter_from_user_mode() changes the system call number to an invalid number and pt_regs->ax to !-ENOSYS then the system call will return a different value(!) depending on if it is out of range for the table (whatever was poked into pt_regs->ax) or if it corresponds to a hole in the table. This seems to me at least to be The Wrong Thing.
> I think you’re right.
>> Calling regs->ax = sys_ni_syscall() in an else clause would arguably be the right thing here, except possibly in the case where nr (or (int)nr, see below) == -1 or < 0.
> I think the check should be -1 for 64 bit but (u32)nr == (u32)-1 for the 32-bit path. Does that seem reasonable?

I'm thinking overall that depending on 64-bit %rax is once again a
mistake; I realize that the assembly code that did that kept breaking
because people messed with it, but we still have:

* Only the low 32 bits of orig_ax are meaningful, so we return int.
* This importantly ignores the high bits on 64-bit, so comparisons
* sign-extend the low 32 bits.
static inline int syscall_get_nr(struct task_struct *task, struct
pt_regs *regs)
return regs->orig_ax;

"Different interpretation of the same data" is a notorious security
trap. Zero-extending orig_ax would cause different behavior on 32 and 64
bits and differ from the above, so I'm thinking that just once and for
all defining the system call number as a signed int for all the x86 ABIs
would be the sanest.

It still doesn't really answer the question if "movq $-1,%rax; syscall"
or "movl $-1,%eax; syscall" could somehow cause bad things to happen,
though, which makes me a little bit nervous still.


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