SchOS/kernel/sys/sys_mem.c

#include "types.h"
#include "defs.h"
#include "param.h"
#include "stat.h"
#include "memlayout.h"
#include "mmu.h"
#include "spinlock.h"
#include "sleeplock.h"
#include "proc.h"
#include "fs.h"
#include "file.h"
#include "syscall.h"
#include "errno.h"

extern int mappages(pml4e_t *pgdir, void *va, uint64 size, uint64 pa, int perm);

#define MAP_SHARED    0x01
#define MAP_PRIVATE   0x02
#define MAP_FIXED     0x10
#define MAP_ANONYMOUS 0x20
#define PROT_NONE     0
#define PROT_WRITE    0x2

// Reject user mapping requests that fall outside user space. Without this,
// MAP_FIXED at an arbitrary address could create user-accessible PTEs over
// (or vm_unmap_range/kfree pages belonging to) the kernel half of the
// address space.
static int
bad_user_range(uint64 va, uint64 length)
{
    return length == 0 || va + length < va || va + length > KERNBASE;
}

addr_t
sys_sbrk(void)
{
    int n;
    if (argint(0, &n) < 0)
        return -1;
    addr_t addr = myproc()->sz;
    if (growproc(n) < 0)
        return -1;
    return addr;
}

addr_t
sys_brk(void)
{
    addr_t addr;
    if (argaddr(0, &addr) < 0 || addr == 0)
        return myproc()->sz;
    if (addr > myproc()->sz && growproc(addr - myproc()->sz) < 0)
        return myproc()->sz;
    return myproc()->sz;
}

addr_t
sys_mmap(void)
{
    uint64 addr, length, offset;
    int prot, flags, fd;
    struct file *f = 0;

    if (argaddr(0, &addr) < 0 || argaddr(1, &length) < 0 ||
        argint(2, &prot) < 0   || argint(3, &flags) < 0  ||
        argint(4, &fd) < 0     || argaddr(5, &offset) < 0)
        return -1;

    if (fd >= 0 && fd < NOFILE)
        f = myproc()->ofile[fd];

    length = PGROUNDUP(length);
    uint64 va = addr;
    if (length == 0 || length > KERNBASE)
        return -EINVAL;

    if (flags & MAP_FIXED) {
        if (bad_user_range(va, length) || (va % PGSIZE))
            return -EINVAL;
        vm_unmap_range(myproc()->pgdir, va, length);
        if (prot == PROT_NONE)
            return va;
        for (uint64 a = va; a < va + length; a += PGSIZE) {
            char *mem = kalloc();
            if (!mem) return -1;
            memset(mem, 0, PGSIZE);
            if (mappages(myproc()->pgdir, (void*)a, PGSIZE, V2P((uint64)mem), PTE_W | PTE_U) < 0)
                return -1;
        }
        return va;
    }

    struct vma *v = 0;
    for (int i = 0; i < NVMA; i++) {
        if (!myproc()->vma[i].used) { v = &myproc()->vma[i]; break; }
    }
    if (!v) return -1;

    // Ignore non-zero hint addresses (not MAP_FIXED); always pick a fresh VA.
    // Bump-allocated: each mapping gets its own disjoint, exactly-length-sized
    // range, so a single mmap() can never run into a neighboring live mapping
    // regardless of size (the old scheme assigned a fixed 8MB window per vma[]
    // slot index, which a single allocation bigger than 8MB — e.g. a large
    // realloc() growth — could map straight past, panicking mappages("remap")
    // when it hit an already-present PTE in the next slot's territory).
    // munmap()'d ranges are not reclaimed for reuse; that's fine at this scale.
    if (myproc()->mmap_next == 0)
        myproc()->mmap_next = 0x40000000;
    va = myproc()->mmap_next;
    if (bad_user_range(va, length))
        return -ENOMEM;
    myproc()->mmap_next += length;

    int is_fb = 0;
    int shmid = -1;
    // File-backed mapping of a regular file.
    //
    // This used to be ignored entirely: any file mapping silently returned
    // zeroed anonymous pages, so a program that mmap'd a font or a config
    // file got a buffer full of nothing and no error to explain it. Reading
    // the contents in is not demand paging and never will be without a page
    // cache -- but it is the difference between a wrong answer and a right
    // one.
    //
    // MAP_SHARED that is also writable is refused rather than faked. Its
    // whole contract is that stores land in the file and other mappers see
    // them, and there is nothing here that could make that true; quietly
    // giving back a private copy would be the same lie in a new place.
    if (f && f->type == FD_INODE && f->ip->type != T_DEV) {
        if ((flags & MAP_SHARED) && (prot & PROT_WRITE))
            return -ENODEV;
        if (!f->readable)
            return -EACCES;

        for (uint64 a = va; a < va + length; a += PGSIZE) {
            char *mem = kalloc();
            if (!mem) return -ENOMEM;
            memset(mem, 0, PGSIZE);
            if (mappages(myproc()->pgdir, (void*)a, PGSIZE, V2P((uint64)mem), PTE_W | PTE_U) < 0) {
                kfree(mem);
                return -ENOMEM;
            }
        }

        // Read through the file's own offset-independent path, so this does
        // not disturb a descriptor the caller is also reading normally.
        ilock(f->ip);
        readi(f->ip, 0, va, offset, length);
        iunlock(f->ip);

        v->addr = va; v->length = length; v->used = 1;
        v->no_free = 0; v->shmid = -1;
        return va;
    }

    if (f && f->type == FD_INODE && f->ip->type == T_DEV && f->ip->major == 2) {
        // Map the framebuffer where it actually is. This used to map a
        // hardcoded 0xfd000000 for a hardcoded length, so a request for more
        // than the screen holds handed out user-writable PTEs to whatever
        // MMIO sits above the framebuffer, and any machine that put the LFB
        // somewhere else got a window onto the wrong device entirely.
        uint64 fb   = vga_fb_phys();
        uint64 fbsz = PGROUNDUP(vga_fb_size());
        if (fb == 0 || fbsz == 0)
            return -ENODEV;
        if (length > fbsz)
            length = fbsz;
        if (mappages(myproc()->pgdir, (void*)va, length, fb, PTE_W | PTE_U) < 0)
            return -1;
        is_fb = 1;
        // From here the drawing happens with no further syscalls, so this is
        // the kernel's only chance to notice that the screen no longer belongs
        // to the text console. fbclose() puts it back.
        fb_mapped();
    } else {
        for (uint64 a = va; a < va + length; a += PGSIZE) {
            char *mem = kalloc();
            if (!mem) return -1;
            memset(mem, 0, PGSIZE);
            if (mappages(myproc()->pgdir, (void*)a, PGSIZE, V2P((uint64)mem), PTE_W | PTE_U) < 0)
                return -1;
        }
    }

    v->addr = va; v->length = length; v->used = 1;
    v->no_free = is_fb; v->shmid = shmid;
    return va;
}

addr_t
sys_munmap(void)
{
    uint64 addr, length;
    if (argaddr(0, &addr) < 0 || argaddr(1, &length) < 0)
        return -1;
    length = PGROUNDUP(length);
    if (bad_user_range(addr, length))
        return -EINVAL;
    for (int i = 0; i < NVMA; i++) {
        if (myproc()->vma[i].used && myproc()->vma[i].addr == addr) {
            if (myproc()->vma[i].shmid >= 0)
                shm_detach(myproc()->vma[i].shmid);
            if (myproc()->vma[i].no_free)
                vm_unmap_norelease(myproc()->pgdir, addr, myproc()->vma[i].length);
            else
                vm_unmap_range(myproc()->pgdir, addr, myproc()->vma[i].length);
            myproc()->vma[i].used = myproc()->vma[i].addr = myproc()->vma[i].length = 0;
            myproc()->vma[i].no_free = 0;
            myproc()->vma[i].shmid = -1;
            return 0;
        }
    }
    // Partial / untracked unmap — free pages anyway
    vm_unmap_range(myproc()->pgdir, addr, length);
    return 0;
}

addr_t sys_mprotect(void) { return 0; }

// ---------------------------------------------------------------------------
// Shared memory
//
// Two processes mapping the same physical pages. This is what a display
// server and its clients need: without it every frame is a copy through a
// pipe, and the copy is the entire cost of drawing anything.
//
// Deliberately simple. Pages are allocated up front rather than on demand,
// there is no swapping, and a segment lives until the last attachment goes
// away. What it does have is the property that matters: a store by one
// process is visible to the other, because it is the same page.
// ---------------------------------------------------------------------------

#define NSHM        16          // segments
#define SHM_MAXPAGES 512        // 2 MB each -- a 800x600x32 surface is 1.9 MB
#define IPC_PRIVATE  0
#define IPC_CREAT    01000
#define IPC_EXCL     02000

struct shmseg {
    int    used;
    int    key;
    uint64 size;                // rounded up to a page
    int    npages;
    int    nattach;             // live attachments across all processes
    char  *pages[SHM_MAXPAGES]; // kernel virtual addresses
};

static struct shmseg shm[NSHM];
static struct spinlock shmlock;
static int shm_ready;

static void
shm_init_once(void)
{
    if (!shm_ready) {
        initlock(&shmlock, "shm");
        shm_ready = 1;
    }
}

static void
shm_free(struct shmseg *s)
{
    for (int i = 0; i < s->npages; i++)
        if (s->pages[i])
            kfree(s->pages[i]);
    memset(s, 0, sizeof(*s));
}

// fork() gives the child its own attachment to the same segment. Without
// this the count would say one attachment while two processes had it mapped,
// and the first to exit would free the pages under the other.
void
shm_ref(int id)
{
    if (id < 0 || id >= NSHM)
        return;
    shm_init_once();
    acquire(&shmlock);
    if (shm[id].used)
        shm[id].nattach++;
    release(&shmlock);
}

// Called from munmap and from exit, once per detached mapping.
void
shm_detach(int id)
{
    if (id < 0 || id >= NSHM)
        return;
    shm_init_once();
    acquire(&shmlock);
    if (shm[id].used && shm[id].nattach > 0) {
        if (--shm[id].nattach == 0)
            shm_free(&shm[id]);
    }
    release(&shmlock);
}

addr_t
sys_shmget(void)
{
    int key, flags;
    addr_t size;

    if (argint(0, &key) < 0 || argaddr(1, &size) < 0 || argint(2, &flags) < 0)
        return -EINVAL;

    size = PGROUNDUP(size);
    if (size == 0 || size > (uint64)SHM_MAXPAGES * PGSIZE)
        return -EINVAL;

    shm_init_once();
    acquire(&shmlock);

    if (key != IPC_PRIVATE) {
        for (int i = 0; i < NSHM; i++) {
            if (shm[i].used && shm[i].key == key) {
                if ((flags & IPC_CREAT) && (flags & IPC_EXCL)) {
                    release(&shmlock);
                    return -EEXIST;
                }
                if (shm[i].size < size) {
                    release(&shmlock);
                    return -EINVAL;
                }
                release(&shmlock);
                return i;
            }
        }
        if (!(flags & IPC_CREAT)) {
            release(&shmlock);
            return -ENOENT;
        }
    }

    for (int i = 0; i < NSHM; i++) {
        if (shm[i].used)
            continue;
        struct shmseg *s = &shm[i];
        s->npages = (int)(size / PGSIZE);
        for (int pg = 0; pg < s->npages; pg++) {
            s->pages[pg] = kalloc();
            if (!s->pages[pg]) {
                for (int j = 0; j < pg; j++) kfree(s->pages[j]);
                memset(s, 0, sizeof(*s));
                release(&shmlock);
                return -ENOMEM;
            }
            // Zeroed, because a fresh segment handing out the last tenant's
            // bytes would leak them between processes.
            memset(s->pages[pg], 0, PGSIZE);
        }
        s->used = 1;
        s->key  = key;
        s->size = size;
        s->nattach = 0;
        release(&shmlock);
        return i;
    }

    release(&shmlock);
    return -ENOSPC;
}

addr_t
sys_shmat(void)
{
    int id, flags;
    addr_t addr;

    if (argint(0, &id) < 0 || argaddr(1, &addr) < 0 || argint(2, &flags) < 0)
        return -EINVAL;
    if (id < 0 || id >= NSHM)
        return -EINVAL;

    shm_init_once();
    acquire(&shmlock);
    if (!shm[id].used) {
        release(&shmlock);
        return -EINVAL;
    }
    uint64 len = shm[id].size;
    int npages = shm[id].npages;
    shm[id].nattach++;                  // claimed before the lock is dropped
    release(&shmlock);

    struct vma *v = 0;
    for (int i = 0; i < NVMA; i++)
        if (!myproc()->vma[i].used) { v = &myproc()->vma[i]; break; }
    if (!v) { shm_detach(id); return -ENOMEM; }

    // Ignore the hint and take a fresh range, the same way mmap does.
    if (myproc()->mmap_next == 0)
        myproc()->mmap_next = 0x40000000;
    uint64 va = myproc()->mmap_next;
    if (bad_user_range(va, len)) { shm_detach(id); return -ENOMEM; }
    myproc()->mmap_next += len;

    for (int pg = 0; pg < npages; pg++) {
        if (mappages(myproc()->pgdir, (void*)(va + (uint64)pg * PGSIZE), PGSIZE,
                     V2P((uint64)shm[id].pages[pg]), PTE_W | PTE_U) < 0) {
            vm_unmap_norelease(myproc()->pgdir, va, (uint64)pg * PGSIZE);
            shm_detach(id);
            return -ENOMEM;
        }
    }

    v->addr = va; v->length = len; v->used = 1;
    v->no_free = 1;                     // the segment owns these pages
    v->shmid = id;
    return va;
}

// Drop every mapping this process holds that it does not own the pages of,
// before its page tables are torn down.
//
// freevm() frees any mapped page below PHYSTOP, which is right for the heap
// and the stack and wrong for anything shared. The framebuffer survived only
// because it sits above PHYSTOP and was skipped by accident; shared memory is
// kalloc'd, sits below it, and would be handed to kfree while another process
// was still using it -- the worst kind of bug, silent and remote.
//
// Called from exit() and from exec() before the old address space goes away.
void
proc_release_shared(struct proc *p)
{
    for (int i = 0; i < NVMA; i++) {
        if (!p->vma[i].used || !p->vma[i].no_free)
            continue;
        vm_unmap_norelease(p->pgdir, p->vma[i].addr, p->vma[i].length);
        if (p->vma[i].shmid >= 0)
            shm_detach(p->vma[i].shmid);
        p->vma[i].used = 0;
        p->vma[i].no_free = 0;
        p->vma[i].shmid = -1;
    }
}

addr_t
sys_shmdt(void)
{
    addr_t addr;
    if (argaddr(0, &addr) < 0)
        return -EINVAL;

    for (int i = 0; i < NVMA; i++) {
        if (myproc()->vma[i].used && myproc()->vma[i].addr == addr &&
            myproc()->vma[i].shmid >= 0) {
            vm_unmap_norelease(myproc()->pgdir, addr, myproc()->vma[i].length);
            shm_detach(myproc()->vma[i].shmid);
            myproc()->vma[i].used = myproc()->vma[i].addr = myproc()->vma[i].length = 0;
            myproc()->vma[i].no_free = 0;
            myproc()->vma[i].shmid = -1;
            return 0;
        }
    }
    return -EINVAL;
}

// Memory advice/hints — all no-ops; kernel decides on its own.
addr_t sys_madvise(void)   { return 0; }
addr_t sys_msync(void)     { return 0; }
addr_t sys_mincore(void)   { return 0; }
addr_t sys_mlock(void)     { return 0; }
addr_t sys_munlock(void)   { return 0; }
addr_t sys_mlockall(void)  { return 0; }
addr_t sys_munlockall(void){ return 0; }
addr_t sys_fadvise64(void) { return 0; }

// mremap: we don't support region relocation; return ENOMEM.
addr_t sys_mremap(void) { return -ENOMEM; }

// Resource limits: always report unlimited.
addr_t sys_setrlimit(void) { return 0; }

// Process priority: always report 0.
addr_t sys_getpriority(void) { return 0; }
addr_t sys_setpriority(void) { return 0; }