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Feedback-ID: iee494631:Fastmail
Subject: Re: [geda-user] Changing gschem backup file permissions
To: geda-user AT delorie DOT com
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From: "Girvin Herr (gherrl2 AT fastmail DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Message-ID: <220e2113-ca15-9f9b-107b-b9609d3cc2ef@fastmail.com>
Date: Wed, 29 Mar 2023 10:52:37 -0700
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On 3/27/23 6:13 PM, Roland Lutz wrote:
> On Mon, 27 Mar 2023, Girvin Herr (gherrl2 AT fastmail DOT com) [via 
> geda-user AT delorie DOT com] wrote:
>> That is what I was afraid of. I was hoping it could be done in a 
>> configuration file somewhere. Thanks for the info, though.
>
> Well ... if you don't want do re-build gEDA/gaf from source, you could 
> always binary-patch the library.
>
> Go to the directory (e.g., /usr/local) where you installed gEDA/gaf. 
> Make a backup copy of your libgeda library:
>
> $ cp -i lib/libgeda.so.47.0.0 lib/libgeda.so.47.0.0.backup
>
> Look at the disassembled library to find the relevant code location:
>
> $ objdump -d lib/libgeda.so.47.0.0 | less
>
> You can use "/f_save<RET>" to search for the function name and "n" to 
> skip over the procedure linkage table entry (we are not interested in 
> that) until you get to the actual function.  This should look 
> something like this:
>
> 0000000000023a20 <f_save>:
>    23a20:       41 57                   push   %r15
>    23a22:       41 56                   push   %r14
>    23a24:       41 55                   push   %r13
> (and so on)
>
> Now, look for the AND instruction ("/and<RET>") that clears the write 
> bits in the permissions (ug-wx o-x = 0446 = 0x126). There are two AND 
> instructions in f_save; in my case, the relevant one is the second 
> one. It should look something like this:
>
>    23e44:       31 ff                   xor    %edi,%edi
>    23e46:       e8 15 1a ff ff          callq  15860 <umask AT plt>
>    23e4b:       48 8b 7c 24 10          mov    0x10(%rsp),%rdi
>    23e50:       89 c6                   mov    %eax,%esi
>    23e52:       89 44 24 1c             mov    %eax,0x1c(%rsp)
>    23e56:       f7 d6                   not    %esi
>    23e58:       81 e6 26 01 00 00       and    $0x126,%esi
>    23e5e:       e8 1d f2 fe ff          callq  13080 <chmod AT plt>
>
> The previous umask is stored in %esi, inverted, masked with 0x126, and 
> passed to chmod.  We want to change the permission mask, so it is the 
> address of the AND instruction we are interested in (0x23e58 in my 
> case, but yours will be different), as well as the exact machine text 
> (81 e6 26 01 00 00 in my case).
>
> Copy that information, close the disassembly ("q") and run Python.  
> Open the shared library, seek to the position you determined and see 
> if you get the bytes you saw in the disassembly (make sure to enter 
> the extra newline after the third line of code to tell Python the 
> block is over):
>
> $ python3
>>>> with open('lib/libgeda.so.47.0.0', 'rb') as f:
> ...     f.seek(0x23e58)  # <-- replace with your value
> ...     f.read(6)
> ... 147032
> b'\x81\xe6&\x01\x00\x00'
>
> Looks good!  The ampersand is ASCII 0x26, so it's displayed as a 
> visual character, but it is the correct value.  Now replace the mask 
> with the one we want (ugo-x = 0666 = 0x1b6), otherwise using the exact 
> same bytes as seen in your disassembly:
>
>>>> with open('lib/libgeda.so.47.0.0', 'r+b') as f:
> ...     f.seek(0x23e58)  # <-- replace with your value
> ...     f.write(b'\x81\xe6\xb6\x01\x00\x00')  # <-- this too
> ... 147032
> 6
>>>> ^D
>
> (For Python 2, you need to omit the "b" before the byte string.)
>
> You're done!  Now verify that you did indeed change the correct 
> instruction.  You should see exactly one changed byte (from 26 to b6):
>
> $ diff <(objdump -d lib/libgeda.so.47.0.0.backup) <(objdump -d 
> lib/libgeda.so.47.0.0)
> 2c2
> < lib/libgeda.so.47.0.0.backup:     file format elf64-x86-64
> ---
>> lib/libgeda.so.47.0.0:     file format elf64-x86-64
> 19164c19164
> <    23e58:     81 e6 26 01 00 00       and    $0x126,%esi
> ---
>>    23e58:     81 e6 b6 01 00 00 and    $0x1b6,%esi
>
> If this is what you see, the binary patch has succeeded, and you 
> should have the file permissions you want the next time you run 
> gschem.  If you see changes different than that (or no changes at 
> all), DELETE the botched library and restore it from the backup.
>
> Hope that helps!
>
> Roland
>
Roland,

Thanks for the info. I may take a look at it, but I prefer to use the 
unchanged package from gEDA.

I have been giving my problem some thought and I think it will be better 
to create a script to change all 4xx file permissions to 6xx. That 
should catch all of the files that are giving me problems, including any 
non-gschem files, or even files located in a directory other than the 
one I invoke gschem from. It sounds better to me.

Thanks again.

Girvin