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[kernel] Fix reading from empty board buffer (sf63.2)

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Syscall 63.2, reading from the board buffer, was overwriting registers
edx and ebp when the buffer was empty. Now it sets eax and ebx according
to the kernel API.

Because of this ancient bug BOARD checks the status of sf63.2 syscall
this way:
        mcall   63, 2
        cmp     ebx, 1
        jne     no_data
        ; when the buffer is empty, ebx is untouched, i.e. still 2, haha
        ; edx and ebp are destroyed, the code is lucky not to use them

By the way, the bug was found using umka tool.

git-svn-id: svn://kolibrios.org@9897 a494cfbc-eb01-0410-851d-a64ba20cac60
This commit is contained in:
Ivan Baravy 2023-01-30 03:33:03 +00:00
parent 4c20c082c1
commit 59315b183d
1 changed files with 9 additions and 11 deletions
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18
kernel/trunk/kernel.asm
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@ -3393,7 +3393,6 @@ if 1
align 4
delay_hs: ; delay in 1/100 secs
; ebx = delay time
pushad
push ebx
xor esi, esi
@ -4226,24 +4225,23 @@ end if
pop ebx eax
ret
@@:
mov [esp + SYSCALL_STACK.eax], ecx
mov [esp+20], ecx
jmp .ret
.read:
cmp eax, 2
jne .ret
test ecx, ecx
jz @b
add esp, 8 ; returning data in ebx and eax, so no need to restore them
test ecx, ecx
jnz @f
mov [esp + SYSCALL_STACK.eax], ecx
mov [esp + SYSCALL_STACK.ebx], ecx
ret
@@:
mov eax, msg_board_data+1
mov ebx, msg_board_data
movzx edx, byte [ebx]
call memmove
dec [msg_board_count]
mov [esp + SYSCALL_STACK.eax], edx ;eax
mov [esp + 20], dword 1
mov [esp + SYSCALL_STACK.eax], edx
mov [esp + SYSCALL_STACK.ebx], 1
ret
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