Syscall troubles

[dak91]

I implemented gdt idt isr and irq following Bran's tutorial (http://www.osdever.net/bkerndev/), now, I've started a syscall interface.

This is my C syscall handler: (only a piece)

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    void syscall_callback(regs_t *r)
    {   
       printf("Syscall: %d -> ", r->eax);
       if(r->eax == SYSCALL_FS_FOPEN)         { ...

called from an assembler function:

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    global syscallisr

    extern syscall_callback

    syscallisr:
       call syscall_callback
       iret

and the handler was added with this call:

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    idt_set_gate(84, (unsigned)syscallisr, 0x08, 0x8E);

I wanted to try if syscalls correctly run. I only put 1 on eax, and call the interrupt of syscall:

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    asm volatile("mov $1,%eax\n int $84\n");

but into the C syscall handler, (regs_t *)->eax doesn't have the value that I putted with mov, why? Sorry for my bad english xD

[pcmattman]

called from an assembler function:

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    global syscallisr

    extern syscall_callback

    syscallisr:
       call syscall_callback
       iret

and the handler was added with this call:

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    idt_set_gate(84, (unsigned)syscallisr, 0x08, 0x8E);

You may want to try pushing the general purpose registers and segments to actually build your stack frame before you call "syscall_callback". An interrupt like the one you're doing only pushes CS:EIP and the EFLAGS (and possibly SS:ESP, if a privilege level change occurs - see section 5.12.1, Volume 3A, of the Intel Manuals).

[Brendan]

Hi,

Your "syscall_callback(regs_t *r)" function expects the real EAX (the first and only parameter) to be a pointer to a structure. I would assume that your ISR handler is meant to build this structure on the stack (e.g. by using PUSH and maybe PUSHA), then set EAX to the address of this structure before calling the C code. For the C code, "r->eax" would access the copy of EAX in this structure, that was saved on the stack by "syscallisr". I'd also assume that your ISR handler is meant to load values from the stack back into general registers before it returns (e.g. with POP and maybe POPA).

For example, if might go something like:

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syscallisr:
       push ds
       push es
       pusha
       mov eax,<kernel's_data_selector>
       mov ds,eax
       mov es,eax
       mov eax,esp                    ;eax = pointer to the structure on the stack
       call syscall_callback
       popa
       pop es
       pop ds
       iret

Of course what you put on the stack (and what order you do it) must match the fields in your "regs_t" structure, and must match what you take off of the stack before you IRET.


Cheers,

Brendan

[Velko]

Your "syscall_callback(regs_t *r)" function expects the real EAX (the first and only parameter) to be a pointer to a structure.

Am I missing something? Shouldn't pointer to structure be pushed onto stack?

[Brendan]

Hi,

Your "syscall_callback(regs_t *r)" function expects the real EAX (the first and only parameter) to be a pointer to a structure.

Am I missing something? Shouldn't pointer to structure be pushed onto stack?

Ah - yes

I'm used to my own calling conventions (assembly language)...


Cheers,

Brendan