Fix disable_colors formatting & test ctx disasm showing fds

This commit adds a test for context disasm showing of file descriptors
file paths in syscalls like read() or close().

It also fixes a small issue when Pwndbg is run with PWNDBG_DISABLE_COLORS=1

This issue was that executing:
```
pi '{a:2}'.format(a=pwndbg.color.context.prefix(pwndbg.config.code_prefix))
```

Failed when Pwndbg was run with disabled colors. It failed because our
generate color functions in pwndbg/color/* ended up not processing the
input argument -- which here is a Pwndbg config Paramater object -- so
that we got a very non obvious exception:
```
Exception occurred: context: unsupported format string passed to Parameter.__format__ (<class 'TypeError'>)
```

This issue could hypothetically also exist if our config value would be
empty I think. So with the fix in this commit, where we do str(x) over
the color funciton argument should fix this issue in all cases.

pwndbg/color/__init__.py

@@ -131,7 +131,12 @@ def generateColorFunctionInner(old, new):
 
 
 def generateColorFunction(config):
-    function = lambda x: x
+    # the `x` here may be a config Parameter object
+    # and if we run with disable_colors or if the config value
+    # is empty, we need to ensure we cast it to string
+    # so it can be properly formatted e.g. with:
+    # "{config_param:5}".format(config_param=some_config_parameter)
+    function = lambda x: str(x)
 
     if disable_colors:
         return function

pwndbg/commands/context.py

@@ -655,7 +655,7 @@ def get_filename_and_formatted_source():
     source = source[start:end]
 
     # Compute the prefix_sign length
-    prefix_sign = pwndbg.config.code_prefix
+    prefix_sign = C.prefix(str(pwndbg.config.code_prefix))
     prefix_width = len(prefix_sign)
 
     # Format the output
@@ -666,7 +666,7 @@ def get_filename_and_formatted_source():
             fmt = C.highlight(fmt)
 
         line = fmt.format(
-            prefix_sign=C.prefix(prefix_sign) if line_number == closest_line else "",
+            prefix_sign=prefix_sign if line_number == closest_line else "",
             prefix_width=prefix_width,
             line_number=line_number,
             num_width=num_width,

tests/binaries/use-fds.c

@@ -0,0 +1,17 @@
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+int main(int argc, char* argv[]) {
+	char buf[16] = {0};
+
+	// read 0 bytes so it won't block
+	read(STDOUT_FILENO, buf, 0);
+
+	int fd = open(argv[0], 0);
+
+	read(fd, buf, sizeof(buf));
+
+	close(fd);
+}

tests/test_context_commands.py

@@ -0,0 +1,63 @@
+import re
+
+import gdb
+
+import pwndbg.commands
+import tests
+
+USE_FDS_BINARY = tests.binaries.get("use-fds.out")
+
+
+def test_context_disasm_show_fd_filepath(start_binary):
+    """
+    Tests context disasm command and whether it shows properly opened fd filepath
+    """
+    start_binary(USE_FDS_BINARY)
+
+    # Run until main
+    gdb.execute("break main")
+    gdb.execute("continue")
+
+    # Stop on read(0, ...) -> should show /dev/pts/X
+    gdb.execute("nextcall")
+
+    out = pwndbg.commands.context.context_disasm()
+    assert "[ DISASM / x86-64 / set emulate on ]" in out[0]  # Sanity check
+
+    call_read_line_idx = out.index(next(line for line in out if "<read@plt>" in line))
+    lines_after_call_read = out[call_read_line_idx:]
+
+    line_call_read, line_fd, line_buf, line_nbytes, *_rest = lines_after_call_read
+
+    assert "call   read@plt" in line_call_read
+
+    line_fd = line_fd.strip()
+    assert re.match(r"fd:\s+0x1 \(/dev/pts/\d+\)", line_fd)
+
+    line_buf = line_buf.strip()
+    assert re.match(r"buf:\s+0x[0-9a-f]+ ◂— 0x0", line_buf)
+
+    line_nbytes = line_nbytes.strip()
+    assert re.match(r"nbytes:\s+0x0", line_nbytes)
+
+    # Stop on open(...)
+    gdb.execute("nextcall")
+    # Stop on read(...) -> should show use-fds.out
+    gdb.execute("nextcall")
+
+    out = pwndbg.commands.context.context_disasm()
+    assert "[ DISASM / x86-64 / set emulate on ]" in out[0]  # Sanity check
+
+    call_read_line_idx = out.index(next(line for line in out if "<read@plt>" in line))
+    lines_after_call_read = out[call_read_line_idx:]
+
+    line_call_read, line_fd, line_buf, line_nbytes, *_rest = lines_after_call_read
+
+    line_fd = line_fd.strip()
+    assert re.match(r"fd:\s+0x3 \([a-z/]*pwndbg/tests/binaries/use-fds.out\)", line_fd)
+
+    line_buf = line_buf.strip()
+    assert re.match(r"buf:\s+0x[0-9a-f]+ ◂— 0x0", line_buf)
+
+    line_nbytes = line_nbytes.strip()
+    assert re.match(r"nbytes:\s+0x10", line_nbytes)