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https://github.com/jart/cosmopolitan.git
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f4f4caab0e
I wanted a tiny scriptable meltdown proof way to run userspace programs and visualize how program execution impacts memory. It helps to explain how things like Actually Portable Executable works. It can show you how the GCC generated code is going about manipulating matrices and more. I didn't feel fully comfortable with Qemu and Bochs because I'm not smart enough to understand them. I wanted something like gVisor but with much stronger levels of assurances. I wanted a single binary that'll run, on all major operating systems with an embedded GPL barrier ZIP filesystem that is tiny enough to transpile to JavaScript and run in browsers too. https://justine.storage.googleapis.com/emulator625.mp4
125 lines
6 KiB
C
125 lines
6 KiB
C
/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
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│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8 :vi│
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╞══════════════════════════════════════════════════════════════════════════════╡
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│ Copyright 2020 Justine Alexandra Roberts Tunney │
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│ │
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│ This program is free software; you can redistribute it and/or modify │
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│ it under the terms of the GNU General Public License as published by │
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│ the Free Software Foundation; version 2 of the License. │
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│ │
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│ This program is distributed in the hope that it will be useful, but │
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│ WITHOUT ANY WARRANTY; without even the implied warranty of │
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│ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU │
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│ General Public License for more details. │
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│ │
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│ You should have received a copy of the GNU General Public License │
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│ along with this program; if not, write to the Free Software │
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│ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA │
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│ 02110-1301 USA │
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╚─────────────────────────────────────────────────────────────────────────────*/
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#include "libc/calls/calls.h"
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#include "libc/calls/internal.h"
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#include "libc/calls/struct/sigset.h"
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#include "libc/runtime/runtime.h"
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#include "libc/runtime/symbols.h"
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#include "libc/sysv/consts/prot.h"
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/**
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* Rewrites code in memory to hook function calls.
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*
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* We do this by searching each function for the nop instruction
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* inserted by GCC when we use the -pg -mnop-mcount flags. There's no
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* risk of corrupting data since the linker scripts won't mix code and
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* data.
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*
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* Modules built with -O3 and without the profiling flags might have
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* these same nop instructions, but that shouldn't be problematic since
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* they're only there for the puposes of aligning jumps, and therefore
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* aren't actually executed. However codebases that use huge function
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* alignments with wide-nop slides could pose minor issues. Further note
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* that Cosmopolitan sources are almost never intentionally written to
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* use code alignment, since we've only seen a few cases where it helps.
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*
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* @see ape/ape.lds
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*/
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privileged void __hook(void ifunc(void), struct SymbolTable *symbols) {
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size_t i;
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intptr_t addr;
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sigset_t oldmask;
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uint64_t code, mcode;
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unsigned char *p, *pe;
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const intptr_t kMcount = (intptr_t)&mcount;
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const intptr_t kProgramCodeStart = (intptr_t)&_ereal;
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const intptr_t kPrivilegedStart = (intptr_t)&__privileged_start;
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const bool kIsBinaryAligned = !(kPrivilegedStart & (PAGESIZE - 1));
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sigprocmask(SIG_BLOCK, &kSigsetFull, &oldmask);
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if (mprotect((void *)symbols->addr_base,
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kPrivilegedStart - symbols->addr_base,
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kIsBinaryAligned ? PROT_READ | PROT_WRITE
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: PROT_READ | PROT_WRITE | PROT_EXEC) != -1) {
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for (i = 0; i < symbols->count - 1; ++i) {
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if (symbols->addr_base + symbols->symbols[i].addr_rva <
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kProgramCodeStart) {
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continue; /* skip over real mode symbols */
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}
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if (symbols->addr_base + symbols->symbols[i].addr_rva >=
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kPrivilegedStart) {
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break; /* stop before privileged symbols */
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}
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for (p = (unsigned char *)(symbols->addr_base +
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symbols->symbols[i].addr_rva),
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pe = (unsigned char *)(symbols->addr_base +
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symbols->symbols[i + 1].addr_rva);
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p < pe - 8; ++p) {
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code = read64le(p);
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/*
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* Test for -mrecord-mcount (w/ -fpie or -fpic)
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*
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* nopw 0x00(%rax,%rax,1) ← morphed by package.com
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* call *mcount(%rip) ← linked w/o -static
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* addr32 call mcount ← relaxed w/ -static
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* addr32 call mcount ← relaxed w/ -static
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*
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* Note that gcc refuses to insert the six byte nop.
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*/
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if ((code & 0x0000FFFFFFFFFFFF) == 0x0000441F0F66 ||
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(code & 0x0000FFFFFFFFFFFF) ==
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((((kMcount - ((intptr_t)&p[2] + 4)) << 16) | 0xE867) &
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0x0000FFFFFFFFFFFF) ||
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(code & 0x0000FFFFFFFFFFFF) ==
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((((kMcount - ((intptr_t)&p[2] + 4)) << 16) | 0xFF15) &
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0x0000FFFFFFFFFFFF)) {
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p[0] = 0x67;
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p[1] = 0xE8;
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addr = (intptr_t)ifunc - ((intptr_t)&p[2] + 4);
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p[2] = addr >> 000;
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p[3] = addr >> 010;
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p[4] = addr >> 020;
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p[5] = addr >> 030;
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break;
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}
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/*
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* Test for -mnop-mcount (w/ -fno-pie)
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*/
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mcode = code & 0x000000FFFFFFFFFF;
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if ((mcode == 0x00441F0F /* nopl 0x00(%eax,%eax,1) [canonical] */) ||
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(mcode == 0x00041F0F67 /* nopl (%eax,%eax,1) [older gcc] */)) {
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if (p[-1] != 0x66 /* nopw 0x0(%rax,%rax,1) [donotwant] */) {
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p[0] = 0xE8 /* call Jvds */;
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addr = (intptr_t)ifunc - ((intptr_t)&p[1] + 4);
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p[1] = addr >> 000;
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p[2] = addr >> 010;
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p[3] = addr >> 020;
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p[4] = addr >> 030;
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}
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break;
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}
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}
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}
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mprotect((void *)symbols->addr_base, kPrivilegedStart - symbols->addr_base,
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PROT_READ | PROT_EXEC);
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}
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sigprocmask(SIG_SETMASK, &oldmask, NULL);
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}
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