STINNER Victor added the comment:
Oh, I just reproduced the issue on my gdb.py program, which is part of the
python-ptrace program. When searching for a pattern in the memory of another
process, /proc/pid/maps is used to get the list of all memory mappings of this
process. On Linux x64, the last mapping is "ffffffffff600000-ffffffffff601000
r-xp 00000000 00:00 0 [vsyscall]", machine code used for system calls. gdb.py
opens /proc/pid/mem in binary mode and tries to seek at 0xffffffffff600000 but
FileIO.seek() fails because it gets an offset bigger than 2^63, which is seen
as a negative number and FileIO.seek() considers a negative number as an error.
Attached patch fixes this issue by checking not only the result of lseek(), but
also errno:
errno = 0;
Py_BEGIN_ALLOW_THREADS
#if defined(MS_WIN64) || defined(MS_WINDOWS)
res = _lseeki64(fd, pos, how);
#else
res = lseek(fd, pos, how);
#endif
Py_END_ALLOW_THREADS
if (res == (off_t)-1 && errno)
return posix_error();
PyLong_AsUnsignedLong() is tried if PyLong_AsLong() failed with an
OverflowError. I kept PyLong_AsLong() for relative seek with a negative offset,
and I prefer to try first PyLong_AsLong() because the overflow case is very
unlikely compared to negative offsets for relative seek.
The result is always casted to an unsigned number. I never seen a device or
file descriptor supporting negative offset.
I don't know how to test this code path automatically. I only know the
"[vsyscall]" mapping use case on Linux, and it requires to parse
/proc/self/maps which is not trivial and I would prefer sometimes simpler (more
reliable with less maintenance). I propose to not add an unit test, except
someone find how to write a simple and reliable test.
> This is not true of a regular file where I get errno=22
> when seeking to an offset greater than or equal to 2^63.
Same behaviour on Linux: OSError(22, "Invalid argument") when I try file.seek(1
<< 63) on a regular file.
----------
Added file: http://bugs.python.org/file30487/lseek_unsigned.patch
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<http://bugs.python.org/issue12545>
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