[Lldb-commits] [PATCH] D70532: [lldb] Improve/fix base address selection in location lists

Pavel Labath via Phabricator via lldb-commits lldb-commits at lists.llvm.org
Thu Nov 21 04:42:16 PST 2019

labath created this revision.
labath added reviewers: JDevlieghere, aprantl, clayborg.
Herald added a project: LLDB.

Lldb support base address selection entries in location lists was broken
for a long time. This wasn't noticed until llvm started producing these
kinds of entries more frequently with r374600.

In r374769, I made a quick patch which added sufficient support for them
to get the test suite to pass. However, I did not fully understand how
this code operates, and so the fix was not complete. Specifically, what
was lacking was the ability to handle modules which were not loaded at
their preferred load address (for instance, due to ASLR).

Now that I better understand how this code works, I've come to the
conclusion that the current setup does not provide enough information
to correctly process these entries. In the current setup the location
lists were parameterized by two addresses:

- the distance of the function start from the start of the compile unit. The purpose of this was to make the location ranges relative to the start of the function.
- the actual address where the function was loaded at. With this the function-start-relative ranges can be translated to actual memory locations.

The reason for the two values, instead of just one (the load bias) is (I
think) MachO, where the debug info in the object files will appear to be
relative to the address zero, but the actual code it refers to
can be moved and reordered by the linker. This means that the location
lists need to be "linked" to reflect the locations in the actual linked

These two bits of information were enough to correctly process location
lists which do not contain base address selection entries (and so all
entries are relative to the CU base). However, they don't work with
them because, in theory two base address can be completely unrelated (as
can happen for instace with hot/cold function splitting, where the
linker can reorder the two pars arbitrarily).

To fix that, I split the first parameter into two:

- the compile unit base address
- the function start address, as is known in the object file

The new algorithm becomes:

- the location lists are processed as they were meant to be processed. The CU base address is used as the initial base address value. Base address selection entries can set a new base.
- the difference between the "file" and "load" function start addresses is used to compute the load bias. This value is added to the final ranges to get the actual memory location.

This algorithm is correct for non-MachO debug info, as there the
location lists correctly describe the code in the final executable, and
the dynamic linker can just move the entire module, not pieces of it. It
will also be correct for MachO if the static linker preserves relative
positions of the various parts of the location lists -- I don't know
whether it actually does that, but judging by the lack of base address
selection support in dsymutil and lldb, this isn't something that has
come up in the past.

I add a test case which simulates the ASLR scenario and demonstrates
that base address selection entries now work correctly here.

  rG LLVM Github Monorepo



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