Bug#1128397: linux-image-6.18.10+deb14-amd64: open(/proc/$pid/maps) is empty after $pid exec()s, unless you read a partial line from the fd before, in which case it has the rest of the line only

[наб]

Package: linux-image-6.18.10+deb14-amd64
Version: 6.18.10-1
Severity: normal

Dear Maintainer,

I reproduce this on 6.18.10+deb14-amd64 (current nightly d-i),
6.1.0-35-amd64, and 6.7.7-amd64:

        $ cat a.c
        int main() {
                        close(0);
                        open("/proc/self/maps", 0);
                        char    buf[20];
                        read(0, buf, sizeof(buf));
                        execlp("cat", "cat", (void *)0);
        }
        $ cc a.c
        $ ./a.out
        b3000 r--p 00000000 00:8a 816985                     
/home/nabijaczleweli/uwu/otp/a.out
        $ sed -e 's:read://&:' -e s/cat/wc/ a.c > b.c
        $ cc b.c
        $ ./a.out
        0 0 0

You can reproduce this with bash with  wc < /proc/self/maps,
which yields 0 0 0 because bash does fork -> open -> exec.

dash does open -> fork -> exec, and it shows dash's maps.

Arguably bash is more correct, except Linux doesn't implement the file
in a useful way (doesn't implement it correctly(?)).

For non-/self/ I tried:
        1$ echo $$
        1727476
        1$ exec sleep inf   # after the sleep below

        2$ (sleep 10; wc) < /proc/1727476/maps
        0 0 0
and
        1$ echo $$
        1730151
        1$ exec sleep inf   # after the sleep below

        2$ (head -c 32; sleep 10; wc) < /proc/1730151/maps
    55cf54298000-55cf542c7000 r--p 01 4 55
which replicates this directly.

(read()ing a /proc/$pid/maps fd after $pid does exist returns ESRCH,
 which is as-expected I think.)

Best,

-- System Information:
Debian Release: 12.11
  APT prefers oldstable-updates
  APT policy: (500, 'oldstable-updates'), (500, 'oldstable-security'), (500, 
'oldstable-debug'), (500, 'oldstable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 6.1.0-35-amd64 (SMP w/24 CPU threads; PREEMPT)
Kernel taint flags: TAINT_PROPRIETARY_MODULE, TAINT_FIRMWARE_WORKAROUND, 
TAINT_OOT_MODULE, TAINT_UNSIGNED_MODULE
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8), 
LANGUAGE=en_GB:en
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

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