.\" $NetBSD: mremap.2,v 1.7.12.2 2023/07/31 16:02:32 martin Exp $ .\" .\" Copyright (c) 2007 Thomas Klausner and Joerg Sonnenberger .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" ------------------------------------------------------------ .Dd May 2, 2018 .Dt MREMAP 2 .Os .Sh NAME .Nm mremap .Nd re-map a virtual memory address .\" ------------------------------------------------------------ .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In sys/mman.h .Ft void * .Fn mremap "void *oldp" "size_t oldsize" "void *newp" "size_t newsize" \ "int flags" .\" ------------------------------------------------------------ .Sh DESCRIPTION The .Fn mremap function resizes the mapped range (see .Xr mmap 2 ) starting at .Ar oldp and having size .Ar oldsize to .Ar newsize . The following arguments can be .Dv OR Ap ed together in the .Ar flags argument: .Bl -tag -width XXMAPXALIGNEDXnXX .It Dv MAP_ALIGNED Ns Pq Ar n The allocation should be aligned to the given boundary, i.e. ensure that the lowest .Ar n bits of the address are zero. The parameter .Ar n should be the base 2 logarithm 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 platform's page size as returned by .Xr sysconf 3 with the .Dv _SC_PAGESIZE request. .It Dv MAP_FIXED .Ar newp is tried and .Fn mremap fails if that address can't be used as new base address for the range. Otherwise, .Ar oldp and .Ar newp are used as hints for the position, factoring in the given alignment. .It Dv MAP_REMAPDUP Duplicate the mapping. Both address ranges reference the same pages, but can have different protection flags. .El .Sh RETURN VALUES .Fn mremap returns the new address or .Dv MAP_FAILED , if the remap failed. .Sh EXAMPLES The following example program creates a two mappings for the same memory range, one RW- and one R-X. .Bd -literal #include #include #include #include #include #include #include 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\en", 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\en", rv); return EXIT_SUCCESS; } .Ed .Sh COMPATIBILITY The semantics of .Fn mremap differ from the one provided by glibc on Linux in that the .Ar newp argument was added and a different set of .Ar flags are implemented. .Sh ERRORS The .Fn mremap function fails if: .Bl -tag -width Er .It Bq Er E2BIG A request to extend .Fa oldp failed because of address overflow. .It Bq Er EINVAL If .Fa oldp or .Fa newp are not page aligned, or .Fa oldsize or .Fa newsize are not a multiple of the page size, or if .Fa oldp + .Fa oldsize or .Fa newp + .Fa newsize wrap around, or if an invalid alignment was requested in the .Fa flags argument, or if the request was to extend .Fa oldp and the extension address space either did not fit, was already occupied, or had set permissions. .It Bq Er ENOENT If the .Fa oldp segmented was not already mapped. .It Bq Er ENOMEM If there was either no space to allocate/move memory, or if a fixed allocation was requested in the .Fa flags that could not be accommodated. .El .Sh SEE ALSO .Xr mmap 2 , .Xr munmap 2 .Sh HISTORY The .Fn mremap system call appeared in .Nx 5.0 . It was based on the code that supports .Fn mremap compatibility for Linux binaries. .Sh BUGS If a mapping is created by .Xr mmap 2 with .Dv MAP_PRIVATE and then duplicated by .Nm with .Dv MAP_REMAPDUP , calling .Xr fork 2 destroys the coupling between the original and duplicate mapping. Workarounds: .Bl -dash .It Create the original mapping with .Dv MAP_SHARED . .It Use .Xr minherit 2 with .Dv MAP_INHERIT_NONE on the writable mapping. .It Don't use .Xr fork 2 ; use .Xr posix_spawn 2 . .El .Pp See .%U https://gnats.NetBSD.org/55177 for details.