mremap(2) - NetBSD Manual Pages

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MREMAP(2)                 NetBSD System Calls Manual                 MREMAP(2)

mremap -- re-map a virtual memory address
Standard C Library (libc, -lc)
#include <sys/mman.h> void * mremap(void *oldp, size_t oldsize, void *newp, size_t newsize, int flags);
The mremap() function resizes the mapped range (see mmap(2)) starting at oldp and having size oldsize to newsize. The following arguments can be OR'ed together in the flags argument: MAP_ALIGNED(n) The allocation should be aligned to the given bound- ary, i.e. ensure that the lowest n bits of the address are zero. The parameter n should be the base 2 loga- rithm of the desired alignment (e.g., to request alignment to 16K, use 14 as the value for n). The alignment must be equal to or greater than the plat- form's page size as returned by sysconf(3) with the _SC_PAGESIZE request. MAP_FIXED newp is tried and mremap() fails if that address can't be used as new base address for the range. Otherwise, oldp and newp are used as hints for the position, fac- toring in the given alignment. MAP_REMAPDUP Duplicate the mapping. Both address ranges reference the same pages, but can have different protection flags.
mremap() returns the new address or MAP_FAILED, if the remap failed.
The following example program creates a two mappings for the same memory range, one RW- and one R-X. #include <sys/types.h> #include <sys/mman.h> #include <stdio.h> #include <string.h> #include <unistd.h> #include <stdlib.h> #include <err.h> static int return_1(void) { return 1; } static void return_1_end(void) { } static int return_2(void) { return 2; } static void return_2_end(void) { } int main(int argc, char *argv[]) { void *maprw, *maprx; int rv; size_t page = (size_t)sysconf(_SC_PAGESIZE); // Create the first mapping that has no protections, but intended // protections only maprw = mmap(NULL, page, PROT_MPROTECT(PROT_EXEC|PROT_WRITE|PROT_READ), MAP_ANON|MAP_PRIVATE, -1, 0); if (maprw == MAP_FAILED) err(EXIT_FAILURE, "mmap failed"); // Create the second mapping for the same physical space, which // again has no protections. maprx = mremap(maprw, page, NULL, page, MAP_REMAPDUP); if (maprx == MAP_FAILED) err(EXIT_FAILURE, "mremap failed"); // Set the first mapping read/write if (mprotect(maprw, page, PROT_READ|PROT_WRITE) == -1) err(EXIT_FAILURE, "mprotect(rw) failed"); // Set the second mapping read/execute if (mprotect(maprx, page, PROT_READ|PROT_EXEC) == -1) err(EXIT_FAILURE, "mprotect(rx) failed"); #define XS(a) (size_t)((uintptr_t)(a ## _end) - (uintptr_t)(a)) // Copy and run the first function memcpy(maprw, return_1, XS(return_1)); __builtin___clear_cache(maprw, (void *)((uintptr_t)maprw + page)); rv = ((int (*)(void))maprx)(); printf("%d\n", rv); // Copy and run the second function memcpy(maprw, return_2, XS(return_2)); __builtin___clear_cache(maprw, (void *)((uintptr_t)maprw + page)); rv = ((int (*)(void))maprx)(); printf("%d\n", rv); return EXIT_SUCCESS; }
The semantics of mremap() differ from the one provided by glibc on Linux in that the newp argument was added and a different set of flags are implemented.
The mremap() function fails if: [E2BIG] A request to extend oldp failed because of address overflow. [EINVAL] If oldp or newp are not page aligned, or oldsize or newsize are not a multiple of the page size, or if oldp + oldsize or newp + newsize wrap around, or if an invalid alignment was requested in the flags argument, or if the request was to extend oldp and the extension address space either did not fit, was already occu- pied, or had set permissions. [ENOENT] If the oldp segmented was not already mapped. [ENOMEM] If there was either no space to allocate/move memory, or if a fixed allocation was requested in the flags that could not be accommodated.
mmap(2), munmap(2)
The mremap() system call appeared in NetBSD 5.0. It was based on the code that supports mremap() compatibility for Linux binaries.
If a mapping is created by mmap(2) with MAP_PRIVATE and then duplicated by mremap with MAP_REMAPDUP, calling fork(2) destroys the coupling between the original and duplicate mapping. Workarounds: - Create the original mapping with MAP_SHARED. - Use minherit(2) with MAP_INHERIT_NONE on the writable mapping. - Don't use fork(2); use posix_spawn(2). See PR lib/55177: for details. NetBSD 10.99 May 2, 2018 NetBSD 10.99
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