Ubuntu 20不支持MAP_FIXED_NOREPLACE

Question

当我运行这段代码时，我得到了"mmap:操作不受支持“，根据mmap人，这是因为其中一个标志无效(由mmap人验证)“坏”标志是MAP_FIXED_NOREPLACE

#include <fcntl.h>
#include <errno.h>
#include <sys/mman.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char** argv)
{
    int fd_addr = open("test", O_CREAT | O_RDWR);
    if (fd_addr == -1) {
        std::cout << "open: " << strerror(errno) << "\n";
        return EXIT_FAILURE;
    }
    if (ftruncate(fd_addr, 100) == -1) {
        std::cout << "ftruncate: " << strerror(errno) << "\n";
        return EXIT_FAILURE;
    }

    auto mem =mmap((void*)0x7f4b1618a000, 1, PROT_READ | PROT_WRITE, MAP_FIXED_NOREPLACE | MAP_SHARED_VALIDATE | MAP_LOCKED, fd_addr, 0);
    if (mem == MAP_FAILED) {
        std::cout << "mmap: " << strerror(errno) << "\n";
        return EXIT_FAILURE;
    }
}

有没有理智的想法，那会是什么，如何找出问题所在？我使用Ubuntu 20 (内核5.4.0-131-泛型)和g++-11 (与glibc 2.31)。

g++-11 (Ubuntu 11.1.0-1ubuntu1~20.04) 11.1.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


ldd (Ubuntu GLIBC 2.31-0ubuntu9.9) 2.31
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.

将MAP_FIXED_NOREPLACE替换为MAP_FIXED运行得很好。这样编撰：

g++-11 -g0 -Ofast -DNDEBUG -Wall -Werror --std=c++2a -march=native -flto -fno-rtti main.cpp -pthread -lrt

Answer 1

要回答你的问题，唯一的办法就是看看你的来源。摘录自mmap

switch (flags & MAP_TYPE) {
        case MAP_SHARED:
            /*
             * Force use of MAP_SHARED_VALIDATE with non-legacy
             * flags. E.g. MAP_SYNC is dangerous to use with
             * MAP_SHARED as you don't know which consistency model
             * you will get. We silently ignore unsupported flags
             * with MAP_SHARED to preserve backward compatibility.
             */
            flags &= LEGACY_MAP_MASK;
            fallthrough;
        case MAP_SHARED_VALIDATE:
            if (flags & ~flags_mask)
                return -EOPNOTSUPP;

将您的flags与LEGACY_MAP_MASK中的操作进行比较，很明显，MAP_FIXED_NOREPLACE不是LEGACY_MAP_MASK的一部分，后者返回您报告的“不支持的操作”错误。

在这种情况下，MAP_FIXED_NOREPLACE位会产生一个前部额外检查，并以MAP_FIXED的形式运行。

长话短说:在这种情况下，您可以在不失去功能的情况下用MAP_SHARED_VALIDATE替换MAP_SHARED。

执行您建议的操作(MAP_FIXED_NOREPLACE到MAP_FIXED)可能会意外地创建重叠内存映射。