Add support for unexported variables access in live patches #19
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inconstante
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Commit b45f0f3 disabled optimization in the live patch, otherwise the reference variable disappears. |
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Commit d40122a changes the following:
Sorry for flooding the pull request with commits. :B |
So far, none of the live patch examples in the test suite needs access to data objects in the target library, instead, all of them replace functions that only deal with local variables or function arguments (either on the stack or in registers). However, direct access to data objects in the target library is desirable and even required by some live patches. If a live patch needs to access a data object that has been exported by the target library, i.e. a GLOBAL object, the compiler toolchain is able to generate the relocations that the loader needs. However, when the data object has not been exported, i.e. a LOCAL object, the toolchain is unable to generate a reference and fails with an 'undefined reference' error message. To bridge this gap, this patch extends the live patch metadata with information about LOCAL data objects. The information is comprised of the name of the variable in the target library, the name of a reference variable in the live patch, as well as the offsets that these two data objects have within their DSOs. The following example (taken from the new test case added by this patch) of a live patch description file provides a clarification: 1. /run/user/1001/libpulp/tests/.libs/libaccess_livepatch1.so 2. @/run/user/1001/libpulp/tests/.libs/libaccess.so.0 3. banner_set:new_banner_set 4. #banner:ulpr_banner:0000000000004020:0000000000004038 Lines 1 and 2 are not new and inform Libpulp about the in-disk paths to the live patch DSO and to the target library, respectively. Line 3 is also not new, and provides a pair of replaced and replacement functions. Line 4 is new. It is preceded by the '#' character and divided into 4 parts, in a predefined order: 1. the name of the data object in the target library, 'banner'; 2. the name of another data object, 'ulpr_banner', which sits within the live patch object and is used as a reference to 'banner'; 3. the offset of 'banner' from the loading address of the target library; 4. the offset of 'ulpr_banner' from the loading address of the live patch object. Then, when a live patch gets applied, Libpulp uses the aforementioned offsets and makes 'ulpr_banner' point to the loaded address of 'banner', which makes it possible for the live patch to access the data object in the target library. PS: The names of the objects (in the target library and in the live patch) are not really necessary and, in fact, not used by libpulp.so. On the other hand, the test infrastructure (tests/offsets.py) uses these names to automatically create a live patch description file.
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Updated branch against current master. |
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Apart from that, the patch adds new information to the live patch metadata, more specifically, the addresses (offsets from the load address) of data objects, and that means that there is no need to open in-disk files (DSOs) during live patch application to fix references to LOCAL objects.
We should probably copy this mechanism to the other parts of the live patch metadata, so that, in the future, nothing else depends on the target library file being both in-disk and in the exact same version that the target process used (which doesn´t work after package updates).