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vcgencmd: Fix gencmd buffer length and result copy - #229
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The GET_GENCMD_RESULT value buffer is the response word followed by MAX_STRING bytes for the command and result, but buffer_len was set to MAX_STRING. The firmware would then look for the next tag in the last word of the command area, which is uninitialised. Set buffer_len to MAX_STRING + 4.
strncat appends up to n characters and then a terminating zero, so passing the full size of the result buffer could write one byte past its end when the firmware's reply fills it. Pass result_len - 1.
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Two small fixes to gencmd():
word, so the firmware would look for the next tag inside the
uninitialised command area rather than at the end tag.
filling it would write the terminator one byte past the end.
Tested on Pi 4 and Pi 5: output and exit codes match the unpatched
vcgencmd for version, get_config, commands, get_throttled and an
unknown command.