IssueTracker

ASharedMemory can no longer be mapped in child progress

* Are you an Android developer?" (Y/N)
  Yes

* Which Android Developer Preview build are you using? See Settings > About phone > Build number (for example RPP1.200123.000).
  RPB3.200720.005 

* Is this a regression from Android 10 to 11?
  Yes

* What device are you using? (for example, Pixel 3 XL)
   Pixel 2 API 30 emulated 

* What are the steps to reproduce the problem? (Please provide the minimal reproducible test case.)

* Issue Category e.g. Framework (platform), NDK (platform), Hardware (CPU, GPU, Sensor, Camera), ART (platform), Runtime Permissions etc
 NDK

* What was the expected result?
  mmap in child progess success 

*How frequently does this issue occur? (e.g 100% of the time, 10% of the time)
 100% of the time

*Expected output as per the apk shared Relevant logcat output.
Code to reproduce the bug https://drive.google.com/file/d/1RKxloyiFUzVxniUHl_pGwB1C62hLAoKj/view?usp=sharing
Devices below android 11

E/MMap (24004): parent progess create ashmem success!! E/MMap (24004): parent progess mmap success!! E/MMap (24004): fork success on main process E/MMap (24024): fork child success E/MMap (24024): child process --child progess start!!--[/data/app/com.example.testmmap-2/lib/arm//libChild-mmap.so]--[26] E/MMap (24024): child process -- map ashmem region suceess !!!  Device android 11 E MMap : parent progess create ashmem success!! E MMap : parent progess mmap success!! E MMap : fork success on main process E MMap : child progess start!!--[/data/app/~~dwoB2cD-fsG1UYLML_cKLw==/com.example.testmmap-m50nMorgvlONu3i6uNFhJw==/lib/x86//libChild-mmap.so]--[54] E MMap : child process --failed to map ashmem region: Bad file descriptor

* What was the actual result?
   mmap in child progess failed

• Link to captured Android bug report (shared privately in Drive.) https://drive.google.com/file/d/1N-S72JaOEcmWOSRptDSmj9x3SpfoOg_m/view?usp=sharing

Code to reproduce the bug https://drive.google.com/file/d/1RKxloyiFUzVxniUHl_pGwB1C62hLAoKj/view?usp=sharing

Devices below android 11 E/MMap (24004): RegisterNatives--result-0 E/MMap (24004): parent progess create ashmem success!! E/MMap (24004): parent progess mmap success!! E/MMap (24004): fork success on main process E/MMap (24024): fork child success E/MMap (24024): child process --child progess start!!--[/data/app/com.example.testmmap-2/lib/arm//libChild-mmap.so]--[26] E/MMap (24024): child process -- map ashmem region suceess !!!

I will describe the problem in detail， I need parent-child process communication. Use ASharedMemory_create() to create shared memory in the parent process and use mmap mapping. This is successful.

int fd = ASharedMemory_create( "mem", 2 * 1024 * 1024);
base = mmap(nullptr, 2 * 1024 * 1024, PROT_READ | PROT_WRITE, MAP_SHARED,
                fd, 0);

Pass the fd descriptor of ASharedMemory_create to the child process through the exec() command line

    pid_t child = fork();
    if (child == -1) {
    } else if (child == 0) {
      char fdStr[16];
      snprintf(fdStr, 16, "%d", fd);
      execve("",1, fdStr);
    } else {}

and use fd for mmap in child progress, The device below andorid11 is successful, but the bad file descriptor is prompted on the device of andorid11. This usage is not valid in andorid11 (api_level 30)? Is it allowed? Are there any restrictions, such as permissions, and how to communicate with large amounts of data if it is not allowed

// child progress main function
int main(int argc, char** argv)
{
    char *arg0 = argv[0];
    int _fd = strtoint(arg0);
    void *base = mmap(nullptr, IPCSIZE, PROT_READ | PROT_WRITE, MAP_SHARED, _fd, 0);
    if (base == MAP_FAILED) {
        close(_fd);
        LOGE("child process --failed to map ashmem region: %s", strerror(errno));
    } else {
        LOGE("child process -- map ashmem region suceess !!!");
    }

ashmem is opened O_CLOEXEC, and has been since Android Pie. specifically since this change: https://android-review.googlesource.com/c/platform/system/core/+/505196

amusingly, since there was no unit test added for that (nor had there been one for the previous behavior), newer code to use memfd_create(2) to implement ashmem on sufficiently new kernels doesn't set O_CLOEXEC.

so right now, whether an ashmem fd (and thus the result of ASharedMemory_create) is O_CLOEXEC or not will depend on exactly what OS version and what kernel version your code runs on. oops.

i'll fix the memfd_create() variant and add the missing test, and update the ASharedMemory_create documentation to point out this issue.

what you'll need to do though is use fcntl with F_SETFD to clear O_CLOEXEC (https://man7.org/linux/man-pages/man2/fcntl.2.html). that will fix your issue on all Android versions/kernel versions.

Re comment #12, nothing currently uses the memfd_create() code path, even on newer kernels. There's a giant switch (ok, a system property) to move the entire device over to memfd, but we first have to migrate apps off of non-NDK use of ashmem, if that's even possible.

Re comment #12, nothing currently uses the memfd_create() code path, even on newer kernels. There's a giant switch (ok, a system property) to move the entire device over to memfd, but we first have to migrate apps off of non-NDK use of ashmem, if that's even possible.

afaik, that switch says "ro.vndk.version >= 29". so last year's OS release will already be using it, not just this year's. (and it's the vendor code's API version i see being checked, not the app's target API level.)

oh, there are two different checks! weird. okay.

to clarify what jmgao said:

Yes, but the flag for cloexec on fcntl is FD_CLOEXEC (1), not O_CLOEXEC (0x80000), which is for open, so you're clearing the flags.

"please change your code to call F_GETFD and clear the FD_CLOEXEC bit instead, or you might have weird issues on a future OS release long after you've forgotten about this" :-)

I modified the code . Thank you everyone

int flg = fcntl(fd,F_GETFD);
flg &= ~FD_CLOEXEC;
fcntl(fd,F_SETFD,flg) 

For memfd to be used the vndk version must be >= 29 and the use_memfd property must be set to true, it's currently false for all devices.

flame:/ # getprop | grep vndk [ro.product.vndk.version]: [30] [ro.vndk.version]: [30]

flame:/ # getprop | grep memfd
[sys.use_memfd]: [false]

Anyway, it does need to be fixed so that we can eventually set use_memfd to true, so thanks for fixing.

https://android-review.googlesource.com/c/platform/frameworks/native/+/1419344 improves the documentation, and https://android-review.googlesource.com/c/platform/system/core/+/1419610 adds the missing unit test and fixes the memfd_create(2) path so it will pass the unit test too once it's enabled.