Hi David,
On Dienstag, 25. Jänner 2022 05:48:34 CET David Gibson wrote:
> On Sat, Jan 22, 2022 at 11:56:54AM +0100, Luca Weiss wrote:
> > Add a new Python method wrapping fdt_get_path() from the C API.
> >
> > Also add a test for the new method.
> >
> > Signed-off-by: Luca Weiss <luca@xxxxxxxxx>
> > ---
> > Changes in v2:
> > - dynamically increase size of string buffer until it fits.
> >
> > The values are chosen relatively arbitrary, and terminates at lengths
> > over than 4096 as I don't expect paths to be longer and there should
> > be an exit condition to not eat up RAM in case of some bug.
> >
> > pylibfdt/libfdt.i | 27 +++++++++++++++++++++++++++
> > tests/pylibfdt_tests.py | 7 +++++++
> > 2 files changed, 34 insertions(+)
> >
> > diff --git a/pylibfdt/libfdt.i b/pylibfdt/libfdt.i
> > index ac70762..d103bb6 100644
> > --- a/pylibfdt/libfdt.i
> > +++ b/pylibfdt/libfdt.i
> >
> > @@ -443,6 +443,28 @@ class FdtRo(object):
> > """
> > return fdt_get_alias(self._fdt, name)
> >
> > + def get_path(self, nodeoffset):
> > + """Get the full path of a node
> > +
> > + Args:
> > + nodeoffset: Node offset to check
> > +
> > + Returns:
> > + Full path to the node
> > +
> > + Raises:
> > + FdtException if the path is longer than 'size' or other error
> > occurs + """
> > + size = 64
> > + while size < 4096:
> > + ret, path = fdt_get_path(self._fdt, nodeoffset, size)
> > + if ret == -NOSPACE:
> > + size += 64
> > + continue
> > + check_err(ret)
> > + return path
> > + raise ValueError('Node path is unexpectedly long')
>
> Ah... sorry, this isn't quite what I had in mind. The idea of
> resizing the buffer is to avoid having an arbitrary limit, but you
> still have the 4096 byte arbitrary limit here. As Simon suggests, 4k
> would probably be a good *starting* point, rather than an ending
> point. Not only are repeated calls expensive in Python, but
> fdt_get_path() itself is also quite expensive, since it needs to scan
> the dtb from the start. I'd also suggest doubling the buffer size on
> each attempt, rather than increasing linearly.
I do think that there should be a limit *somewhere* instead of hitting OOM if
there would be some bug in the underlying implementation or the file provided?
I'm fine with limiting it at 100MB or whatever but I would rather have a limit
there than not have it.
Once this is clarified I'll make a v3 of this patch.
Regards
Luca
>
> > def parent_offset(self, nodeoffset, quiet=()):
> > """Get the offset of a node's parent
> >
> > @@ -1115,6 +1137,11 @@ typedef uint32_t fdt32_t;
> >
> > }
> >
> > }
> >
> > +%include "cstring.i"
> > +
> > +%cstring_output_maxsize(char *buf, int buflen);
> > +int fdt_get_path(const void *fdt, int nodeoffset, char *buf, int buflen);
> > +
> >
> > /* We have both struct fdt_property and a function fdt_property() */
> > %warnfilter(302) fdt_property;
> >
> > diff --git a/tests/pylibfdt_tests.py b/tests/pylibfdt_tests.py
> > index 5479363..2335a73 100644
> > --- a/tests/pylibfdt_tests.py
> > +++ b/tests/pylibfdt_tests.py
> >
> > @@ -348,6 +348,13 @@ class PyLibfdtBasicTests(unittest.TestCase):
> > self.assertEqual("/subnode@1/subsubnode",
> > self.fdt3.get_alias('ss1'))
> > self.assertEqual("/subnode@1/subsubnode/subsubsubnode",
> > self.fdt3.get_alias('sss1'))>
> > + def testGetPath(self):
> > + """Test for the get_path() method"""
> > + node = self.fdt.path_offset('/subnode@1')
> > + node2 = self.fdt.path_offset('/subnode@1/subsubnode')
> > + self.assertEqual("/subnode@1", self.fdt.get_path(node))
> > + self.assertEqual("/subnode@1/subsubnode",
> > self.fdt.get_path(node2)) +
> >
> > def testParentOffset(self):
> > """Test for the parent_offset() method"""
> > self.assertEqual(-libfdt.NOTFOUND,
