Identical execve causes "Permission denied" for one program but not another

Permission denied shows up when I try to open some attachment in Thunderbird. Yet, it doesn’t look as a Thunderbird bug, because when I strace the process I see it issues the correct call. Yet it fails.

11545 execve("/usr/local/bin/libreoffice6.4", ["libreoffice6.4", "--writer", "file:///tmp/user/1000/via%20alla%20quite%2012%20Laigueglia"], ["SHELL=/bin/bash", "WINDOWID=18874383", "QT_ACCESSIBILITY=1", "XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session0", "XTERM_VERSION=XTerm(344)", ... ]) = -1 EACCES (Permission denied)

Permissions are good, I tested with namei and getfacl. Then I copied the execve instruction from strace -v -s 1024 to a C file and compiled it. In that case, it succeeds. Every time I run it by mouse clicks in Thunderbird it fails, every time I run the following test program it succeeds.

#include <unistd.h>
#include <stdio.h>

int main()
{
    char *path="/usr/local/bin/libreoffice6.4";
    char *argv[] =
    {
        "libreoffice6.4",
        "--writer",
        "file:///tmp/user/1000/via%20alla%20quite%2012%20Laigueglia",
        NULL
    };
    char *envp[] =
    {
        "SHELL=/bin/bash",
        "WINDOWID=18874383",
        "QT_ACCESSIBILITY=1",
        "XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session0",
        "XTERM_VERSION=XTerm(344)",
        // ...
        NULL
    };
    int rtc = execve(path, argv, envp);
    perror("execve failed");
    return rtc;
}

Note that Thunderbird can open a number of attachments correctly. Only LibreOffice and a custom shell program fail, thus far. Both are not part of the stock distribution — Devuan Beowulf — but this cannot explain the failure, can it?

There is no point reporting the bug to Thunderbird, because it is an old 68 version, and the bug may be fixed already or perhaps will disappear spontaneously on the next install. Certainly, I cannot report it if I have no clue of what the heck goes wrong. I don’t understand how come the access is alternately denied and granted.

• I run Thunderbird and the straw man program under the same userid/ session.
• Thunderbird doesn’t seem to call any of trace=setuid,setfsuid,setreuid.
• The x bit is on on each part of the relevant paths.
• I never used ACLs.

Edit: The man page for execve lists four possible reasons for EACCESS:

• Search permission is denied on a component of the path prefix of filename or the name of a script interpreter.
• The file or a script interpreter is not a regular file.
• Execute permission is denied for the file or a script or ELF interpreter.
• The filesystem is mounted noexec.

None of those appears to be true. I complicated the straw man program adding dummy thread_creations and forks to mimic Thunderbird behavior. In particular, running this executable:

-rwxr-xr-x 1 root root 14328 Nov  6  2019 /usr/lib/x86_64-linux-gnu/glib-2.0/gio-launch-desktop

Note that it is part of libglib2.0-0:amd64, not Thunderbird.

Still, I cannot reproduce the failure as experienced by Thunderbird.

Edit2: Submitted the case against libc6 to both Devuan bug 497 and Debian bug 966343