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109 lines
2.9 KiB
109 lines
2.9 KiB
#define _GNU_SOURCE
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#include <stdlib.h>
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#include <stdio.h>
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#include <assert.h>
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#include <string.h>
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#include <unistd.h>
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#include <sys/mman.h>
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#include <limits.h>
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#include <stddef.h>
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#include <cow.h>
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#define box(t) aligned_alloc(_Alignof(t), sizeof(t))
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#if defined(DEBUG) || defined(COW_TRACE)
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#define TRACE(msg, ...) (fprintf(stderr, "[TRACE] %s->%s %d: " msg "\n", __FILE__, __func__, __LINE__, __VA_ARGS__), (void)0)
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#else
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#define TRACE(msg, ...) ((void)0)
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#endif
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struct cow {
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void* origin; // ptr to mapped memory. This *MUST* be the first field and have an offset of 0.
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int fd; // Will be ORd with ~INT_MAX if it's a clone. Will be >0 if it's the original.
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size_t size;
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}; // cow_t, *cow
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_Static_assert(offsetof(cow_t, origin) == 0, "`cow_t.origin` must have an offset of 0.");
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static __attribute__((noreturn)) __attribute__((noinline)) __attribute__((cold)) void die(const char* error)
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{
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perror(error);
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exit(1);
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}
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static inline cow_t* box_value(cow_t v)
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{
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cow_t* boxed = box(cow_t);
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TRACE("boxing cow_t { origin = %p, fd = 0x%x, size = %lu } -> %p (%lu bytes)", v.origin, v.fd, v.size, (const void*)boxed, sizeof(cow_t));
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*boxed = v;
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return boxed;
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}
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static inline int shm_fd(size_t size)
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{
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#ifdef COW_NAME_SHM
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_Thread_local static char buffer[12] = {0};
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snprintf(buffer, 11, "0x%08lx", size);
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//fprintf(stderr, "shm_fd_name: '%s'\n", buffer);
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// XXX: Not entirely sure how ownership works for the string passed here... Let's not.
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int fd = memfd_create(buffer, 0);
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#else
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int fd = memfd_create("cow_create:shm_fd", 0);
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#endif
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if(fd<=0) die("cow_create:shm_fd:memfd_create");
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ftruncate(fd, size);
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return fd;
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}
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static inline int cow_real_fd(const cow_t* cow)
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{
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return cow->fd & INT_MAX;
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}
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int cow_is_fake(const cow_t* cow)
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{
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return cow->fd<0;
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}
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size_t cow_size(const cow_t* cow)
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{
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return cow->size;
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}
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cow_t* cow_create(size_t size)
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{
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cow_t ret;
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ret.size = size;
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ret.fd = shm_fd(size);
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ret.origin = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_SHARED, ret.fd, 0);
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if(ret.origin == MAP_FAILED) die("cow_create:mmap");
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TRACE("mapped new cow page of %lu size at %p (memfd %d)", size, ret.origin, ret.fd);
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return box_value(ret);
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}
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void cow_free(cow_t* restrict cow)
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{
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TRACE("unmapping %s cow of %lu size from %p (fd %d, real fd %d)", cow_is_fake(cow) ? "fake" : "and closing fd of origin", cow->size, cow->origin, cow->fd, cow_real_fd(cow));
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munmap(cow->origin, cow->size);
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if(!cow_is_fake(cow))
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close(cow->fd);
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free(cow);
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}
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cow_t* cow_clone(const cow_t* cow)
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{
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cow_t clone;
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clone.origin = mmap(cow->origin, cow->size, PROT_READ|PROT_WRITE, MAP_PRIVATE, cow_real_fd(cow), 0);
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if(clone.origin == MAP_FAILED) die("cow_clone:mmap");
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clone.fd = (~INT_MAX) | cow->fd;
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clone.size = cow->size;
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TRACE("mapped cloned cow page of %lu size from %p (%s) at %p (clone fd %d, parent fd %d, real fd %d)", clone.size, cow->origin, cow_is_fake(cow) ? "fake" : "origin", clone.origin, clone.fd, cow->fd, cow_real_fd(&clone));
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return box_value(clone);
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}
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