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> Linux > Malloc in assembly, how far can I reach? |
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revolution 08 Jul 2023, 08:54
Unless you statically allocate all memory at the time you generate the exe file, then you will always need to use the OS APIs to allocate memory.
For which OS? For Linux you can use SYS_BRK, MMAP, etc. system calls. For Windows there is VirtualAlloc, etc. For other OS then it could be anything. So you can avoid the stdlib stuff, but it needs to be replaced with some other way to allocate memory. |
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08 Jul 2023, 08:54 |
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tianboh 09 Jul 2023, 01:12
Thank you for your reply! In fact, I am using Linux.
Another question, If I use calloc from libc, can I simply write something like mov $1 %rdi mov $4 %rsi call calloc mov %rax %TARGET_REG to allocate an integer? I tried to generate assembly using GCC, it uses PLT, and generate code like blah blah call malloc@PLT blah blah However, I haven't seen any linker or assembler that can handle PLT in my environment(I am self studying a compiler course, and this is its assignment), so I am not sure how to implement alloc in my compiler. Any ideas? |
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09 Jul 2023, 01:12 |
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revolution 09 Jul 2023, 07:47
For Linux using SYS_BRK is like this:
Code: format ELF64 executable 0 entry main RAM_NEEDED = 10 shl 10 SYS_WRITE = 1 SYS_BRK = 12 SYS_EXIT_GROUP = 231 STD_OUTPUT = 1 segment executable readable main: mov eax,SYS_BRK xor edi,edi syscall lea rdi,[rax + RAM_NEEDED] mov eax,SYS_BRK syscall cmp rax,rdi lea rsi,[mem_good] mov edx,mem_good.len jz .message_okay lea rsi,[mem_bad] mov edx,mem_bad.len .message_okay: mov eax,SYS_WRITE mov edi,STD_OUTPUT syscall mov eax,SYS_EXIT_GROUP xor edi,edi syscall mem_good: db 'Allocation successful',10 mem_good.len = $ - mem_good mem_bad: db 'Allocation failed',10 mem_bad.len = $ - mem_bad |
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09 Jul 2023, 07:47 |
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tianboh 10 Jul 2023, 01:35
wow! That's awesome, still I got some questions
1. Why is syscall having a different calling convention? It takes eax to determine the brk instruction and uses rdi to denote a void pointer. I feel like SYS_BRK/12 should be passed as the first parameter(edi) and address pointer the second(rsi). 2. It seems you allocated twice through syscall. Is there any insight behind it? |
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10 Jul 2023, 01:35 |
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revolution 10 Jul 2023, 02:30
The Linux kernel doesn't use C calling convention. Typing "man syscall" in a terminal shows ...
Code: ~ man syscall ... arch/ABI arg1 arg2 arg3 arg4 arg5 arg6 arg7 Notes ────────────────────────────────────────────────────────────────── arm/OABI a1 a2 a3 a4 v1 v2 v3 arm/EABI r0 r1 r2 r3 r4 r5 r6 arm64 x0 x1 x2 x3 x4 x5 - blackfin R0 R1 R2 R3 R4 R5 - i386 ebx ecx edx esi edi ebp - ia64 out0 out1 out2 out3 out4 out5 - mips/o32 a0 a1 a2 a3 - - - See below mips/n32,64 a0 a1 a2 a3 a4 a5 - parisc r26 r25 r24 r23 r22 r21 - s390 r2 r3 r4 r5 r6 r7 - s390x r2 r3 r4 r5 r6 r7 - sparc/32 o0 o1 o2 o3 o4 o5 - sparc/64 o0 o1 o2 o3 o4 o5 - x86_64 rdi rsi rdx r10 r8 r9 - x32 rdi rsi rdx r10 r8 r9 - |
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10 Jul 2023, 02:30 |
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tianboh 10 Jul 2023, 03:09
Thank you so much for the great explanation! It helps a lot!
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10 Jul 2023, 03:09 |
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