Fix wrong struct layout in getInterfaceSpeedGLinkSettings #382
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The `SIOCETHTOOL` IOCTL expects a pointer to an instance of `ethtool_link_settings`. E.g. it will read the `cmd` member to determine what to do. However facebookincubator#346 reordered the memory layout of the pointer such that actually and array of `__32` values (zeroed out) is passed. Hence the IOCTL will either fail because an invalid command (`cmd=0`) is passed or the values read later by e.g. `ecmd.req.link_mode_masks_nwords` are something completely different. So `ethtool_link_settings` has to come before the (stack) memory used in the flexible array at the end of this struct. To avoid GCC warnigns/errors (see facebookincubator#345) an union is used that provides the current struct (i.e. with wrong order) and an access the actually used `struct ethtool_link_settings` at the top.
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@kumpera Would you be able to review and merge this? CI seems to be broken/unusable right now. @cdluminate If you want/can take a look if this works for you too? |
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The
SIOCETHTOOLIOCTL expects a pointer to an instance ofethtool_link_settings.E.g. it will read the
cmdmember to determine what to do. However #346 reordered the memory layout of the pointer such that actually and array of__32values (zeroed out) is passed. Hence the IOCTL will either fail because an invalid command (cmd=0) is passed or the values read later by e.g.ecmd.req.link_mode_masks_nwordsare something completely different.So
ethtool_link_settingshas to come before the (stack) memory used in the flexible array at the end of this struct.To avoid GCC warnigns/errors (see #345) an union is used that provides the current struct (i.e. with wrong order) and an access the actually used
struct ethtool_link_settingsat the top.Alternative approach to #348 likely avoiding any undefined behavior (union based type punning is usually supported by compilers, so this should work to get the required memory)