Make shell execution a bit more reliable

Previously you would not get feedback when quotes are not terminated or
when the command is too long. Fix this by putting stderr redirection in
the shell command, before eval.

lglaf.py

@@ -126,11 +126,14 @@ def validate_message(payload, ignore_crc=False):
         raise RuntimeError("Expected trailer %r, found %r" % (tail_exp, tail))
 
 def make_exec_request(shell_command):
-    # Allow use of shell constructs such as piping. Needs more work not to eat
-    # all repetitive spaces, it should also escape some things...
-    body = b'sh -c "$@" -- eval 2>&1 </dev/null '
-    body += shell_command.encode('ascii') + b'\0'
-    return make_request(b'EXEC', body=body)
+    # Allow use of shell constructs such as piping and reports syntax errors
+    # such as unterminated quotes. Remaining limitation: repetitive spaces are
+    # still eaten.
+    argv = b'sh -c eval\t"$*"</dev/null\t2>&1 -- '
+    argv += shell_command.encode('ascii')
+    if len(argv) > 255:
+        raise RuntimeError("Command length %d is larger than 255" % len(argv))
+    return make_request(b'EXEC', body=argv + b'\0')
 
 
 ### USB or serial port communication

protocol.md

@@ -130,19 +130,25 @@ reboot, everything was zeroed out though.
 
 ### EXEC - Execute Command
 Arguments: none
-Request body: NUL-terminated command.
+Request body: NUL-terminated command, at most 255 bytes including terminator.
 Response body: standard output of the command.
 
-The command is probably split on space and then passes to `execve`. In order to
-see standard error, use variables and globbing, use a command such as:
+The command is split on spaces and then passed to `execvp`. In order to see
+standard error, use variables and globbing, use a command such as:
 
-    sh -c "$@" -- eval 2>&1 </dev/null echo $PATH
+    sh -c eval\t"$*"</dev/null\t2>&1 -- echo $PATH
+
+(replace `\t` by tabs)
 
 If you need to read dmesg (or other blocking files), try to put busybox on the
 device (e.g. by writing to an unused partition) and execute:
 
     /data/busybox timeout -s 2 cat /proc/kmsg
 
+The maximum output size appears to be 0x800001 (`LAF_MAX_DATA_PAYLOAD`). Larger
+values result in an error. Output is read per byte, not very efficient for large
+output...
+
 ### INFO
 Arguments:
  - arg1: action (`GPRO` - Get Properties, `SPRO` - Set Properties)