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Re: Running GCC as root

Ian Lance Taylor <> writes:

> Zack Weinberg <> writes:
>> $ sudo mknod null c 1 3
>> $ sudo chmod 666 null
>> $ strace as -Z test.s -o null 2>&1 | grep null | grep -v execve
>> stat64("null", {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0
>> open("null", O_RDWR|O_CREAT|O_TRUNC, 0666) = 3
>> so gas does seem to do the right thing in this case.  I also observe
>> the same pattern for ld.
> For the record, this happens because BFD only unlinks the file first
> if the size is non-zero.  BFD considers unlinking the file to avoid
> errors which arise when overwriting a running binary (e.g., on SunOS
> this was permitted by the OS, but could sometimes cause signal
> handlers to fail in the running binary; strange but true). 

Right.  And some OSes won't allow it at all, e.g. HP-UX.  I'm a little
nervous about relying on st_size==0 to catch non-regular files as well
as gcc's temp files; there's no intrinsic reason why *all* non-regular
files should have size zero.

> BFD does not unlink an empty file because gcc will create the .o
> file first in make_temp_file, and unlinking the file would cause the
> same race condition which gcc is trying to avoid.  Of course the
> issue about a running binary normally only arises with ld, but gas
> uses the same routine the open the output file.
> Using O_EXCL in the assembler will fail exactly because this bug was
> fixed in gcc: in the normal case gcc will have created the file
> already.  Although I suppose we could only use O_EXCL in the case
> where the stat failed.

Doh, you're right.  This didn't occur to me because I was running the
assembler by hand.

The same race comes up internally when BFD does unlink the output
file.  I think that using O_EXCL when either the stat failed or BFD
unlinked the file would be enough.

> In general writing to a temporary file and renaming does seem to be
> the right approach here.

Though I suppose there might be problems with another process holding
the file open and expecting the binary to show up there...


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