Issue with getmntent_r with overlay/overlay2 with /proc/mounts entries >=1024 bytes

While investigating an issue with Java and how JNI is using getMountEntries() and it’s getmntent JNI dispatcher we came across a discrepancy between how getmntent_r() behaves in musl and glibc on the following conditions:

• docker storage driver is overlay or overlay2
• there are enough layers so that the overlay / entry in proc mounts in longer than 1024 characters

Note that this is separate from the older Alpine bug#5703 that deals with the 256byte fixed limit.

Here are the reproduction steps:

1. Ensure `docker info` reports overlay/overlay2. This is the default, e.g. on Fedora, Docker for Mac and Docker for Windows.

2. Create the following files in your dir:

cat >docker-compose.yml <<EOF  
version: ‘2’

services:  
mytester:  
build: .  
image: testgetmntent:alpine  
EOF

cat >Dockerfile <<EOF  
FROM alpine:latest  
RUN apk update && apk add bash gcc musl-dev && rm -rf
/var/cache/apk/\*  
RUN echo test 0 >/tmp/test0  
RUN echo test 1 >/tmp/test1  
RUN echo test 2 >/tmp/test2  
RUN echo test 3 >/tmp/test3  
RUN echo test 4 >/tmp/test4  
RUN echo test 5 >/tmp/test5  
RUN echo test 6 >/tmp/test6  
RUN echo test 7 >/tmp/test7  
RUN echo test 8 >/tmp/test8  
RUN echo test 9 >/tmp/test9  
RUN echo test 10 >/tmp/test10  
RUN echo test 11 >/tmp/test11  
EOF

3. Build and run the resulting image with:

docker-compose build —pull && docker-compose run mytester /bin/bash

4. In the shell, create the following C program to test getmntent_r:

cat >getmntent\_r\_tester.c <<EOF  
\#include <stdio.h> 
\#include <mntent.h>

void print\_mount(const struct mntent \*fs);

int main(int argc, char \*\*argv)  
{  
FILE \*fp;  
struct mntent \*fs;  
struct mntent ent;  
char buf\[1024\];  
int buflen = sizeof(buf);

fp = setmntent(“/proc/mounts”, “r”); /\* read only \*/  
if (fp == NULL) {  
printf(“Error calling setmntent\\n”);  
return(1);  
}  
while ((fs = getmntent\_r(fp, &ent, (char\*)&buf, buflen )) != NULL)  
print\_mount(fs);

endmntent(fp);  
}

void print\_mount(const struct mntent \*fs)  
{  
printf(“%s %s %s %s %d %d\\n”,  
fs->mnt\_fsname,  
fs->mnt\_dir,  
fs->mnt\_type,  
fs->mnt\_opts,  
fs->mnt\_freq,  
fs->mnt\_passno);  
}  
EOF

5. Compile and run it. Notice there’s no output.

bash-4.3\# gcc getmntent\_r\_tester.c  
bash-4.3\# ./a.out

6. Notice that overlay / entry in /proc/mounts exceeds 1024bytes:

bash-4.3\# grep overlay /proc/mounts | wc -c  
1069

7. Exit the container, run docker-compose down.

8. Edit the Dockerfile and remove the last 2 RUN commands

9. Rebuild, rerun the image and recompile and rerun the C program, in steps 3-5:

bash-4.3\# gcc getmntent\_r\_tester.c  
bash-4.3\# grep overlay /proc/mounts | wc -c  
961  
bash-4.3\# ./a.out  
overlay / overlay
rw,seclabel,relatime,lowerdir=/var/lib/docker/overlay2/l/LVWNODYSVRXAQW4P5QXNVVXDSS:/var/lib/docker/overlay2/l/RVAZXPKSLDGTV5QXZLI6HZAXXU:/var/lib/docker/overlay2/l/EJIOFNYRMEPTJ4AID2PXUZNJOC:/var/lib/docker/overlay2/l/P4BG3VO46BQDHUXLXPZ3KDXAIR:/var/lib/docker/overlay2/l/G563LG47W5E6A4YPFZCTQ5ORUG:/var/lib/docker/overlay2/l/E2PLIGC66VLBLKENQA2GUKOZXO:/var/lib/docker/overlay2/l/6C5UXCJ4COZVHKHDRDKGW7PKIZ:/var/lib/docker/overlay2/l/GY45BC44L45ZYGQB7XCUU3L4LU:/var/lib/docker/overlay2/l/FF4KRJU7RWCA2GQ6OZV4OHLCXM:/var/lib/docker/overlay2/l/5SZT4SYLCF5LZAZCAJIA6YFB3K:/var/lib/docker/overlay2/l/3SP6Y2WURVZ4L6IZD3WQ2T5ZRG:/var/lib/docker/overlay2/l/LZZGUAD7WB6NZAKG5PSE4K7S6B:/var/lib/docker/overlay2/l/5VA5PMTBPJ4VRPX7HNGUVOYARI,upperdir=/var/lib/docker/overlay2/9ff9f8c3a876cec942dbfa5324894d24d1fe4eb52700320e5e5fa3e7e34a137a/diff,workdir=/var/lib/docker/overlay2/9ff9f8c3a876cec942dbfa5324894d24d1fe4eb52700320e5e5fa3e7e34a137a/work
0 0  

10. Trying the same process with a different docker image base, based on glibc, (e.g. fedora:latest) works in all cases.

(from redmine: issue id 7093, created on 2017-04-05, closed on 2017-06-01)

• Changesets:
  • Revision 189a2712 by Natanael Copa on 2017-05-30T15:58:56Z:

community/openjdk8: increase buffer size for getmntent_r

Java will only use 1024 byte buffer for parsing mounts. Unlike glibc
will musl return error when this is not big enough instead of truncating
it.

We solve it by allocating a much bigger buffer.

fixes #7093

We also build without precompiled headers, which does not work eith PIE.

• Revision d0a7b324 by Natanael Copa on 2018-06-13T21:18:57Z:

community/openjdk8: increase buffer size for getmntent_r

Java will only use 1024 byte buffer for parsing mounts. Unlike glibc
will musl return error when this is not big enough instead of truncating
it.

We solve it by allocating a much bigger buffer.

fixes #7093

We also build without precompiled headers, which does not work eith PIE.