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__INTERNAL_fmt: fix stack offset when output=memory instead of fd

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Kwarde
2025-07-31 14:34:46 +02:00
parent 00568e2e7c
commit be81866768
2 changed files with 15 additions and 4 deletions
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15
src/console.asm
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@@ -123,6 +123,9 @@ eprintf:
; This is because in such I/O functions, first argument should be file descriptor, and following SYS V ABI, that first argument is passed to RDI
; However since __INTERNAL_fmt expects FD via RAX, all arguments have to be shifted. Due to this, argument in R9 becomes a stack argument
; Because of that, __INTERNAL_fmt has to load from stack with a different offset; +24 bytes extra (extra +8 for function call, extra +8 for function prologue and extra +8 to account for possible R9 arg becoming a stack arg)
;
; To make it even more complicated, instead of simply writing to a buffer and simply printing to given FD, it is now also possible to write to memory (array) directly (used for format)
; To achieve that, pass ~RAX (thus ~pointer_to_array) and push length of array to the stack (from within function wrapper). See format()
__INTERNAL_fmt:
%macro load_arg 1
cmp r14, 4
@@ -132,10 +135,18 @@ __INTERNAL_fmt:
%%fromStack:
cmp r15, FD_stderr
jle %%stackNoShift
mov %1, [rbp + (RBP_OFFSET_CALLER*2 + SIZE_QWORD) + ((r14-5) * SIZE_QWORD)]
test rbx, rbx
jnz %%stackShift_format
mov %1, [rbp + (RBP_OFFSET_CALLER*2 + SIZE_QWORD) + ((r14-5) * SIZE_QWORD)] ;Offset when called from wrapper like fwrite() (shifted args, output is to FD)
jmp %%continue
%%stackShift_format:
mov %1, [rbp + (RBP_OFFSET_CALLER*2 + SIZE_QWORD*4) + ((r14-5) * SIZE_QWORD)] ;Offset when called from wrapper like format() (shifted args, output is memory; extra push (+stack offset) for length of array)
;Not entirely sure why I need to do SIZE_QWORD*4 though, should be *3 (because of one push and one stack offset: 16 bytes)
;At %%fromStack I was also not sure why (according to comment at description: '+8 to account for possible R9 arg becoming a stack arg'); this does not apply
;It 1*8 is needed it checks out: 1 + 2 + that extra 1 = 4. Still not sure where the extra 8 is coming from.
jmp %%continue
%%stackNoShift:
mov %1, [rbp + RBP_OFFSET_CALLER + ((r14-5) * SIZE_QWORD)]
mov %1, [rbp + RBP_OFFSET_CALLER + ((r14-5) * SIZE_QWORD)] ;Offset when called from wrapper like printf (no shifted args)
%%continue:
%endmacro

4
src/string.asm
View File

@@ -234,9 +234,9 @@ format:
mov rbp, rsp
mov rax, rdi
not rax ;__INTERNAL_fmt; ~FD => Mem
not rax ;__INTERNAL_fmt; ~RAX => to mem instead of fd
sub rsp, SIZE_QWORD
push rsi ;__INTERNAL_fmt: pop len from stack if rax is signed
push rsi
lea rdi, [rdx]
mov rsi, rcx
mov rdx, r8