main: suggest --os flag when ELF OS detection fails (#2869)

* main: suggest --os flag when OS detection fails for ELF files

When capa cannot detect the target OS of an ELF file, it exits with an
error. Some ELF files lack the standard metadata capa uses for OS
detection (GNU ABI tag, OSABI field, library dependencies, etc.) even
though they do target a valid OS (e.g. a stripped Linux binary using
only raw syscalls).

Add a hint to the unsupported-OS error message telling users they can
specify the OS explicitly with the --os flag, matching the workaround
recommended in the issue.

Fixes #2577

CHANGELOG.md

@@ -20,6 +20,7 @@
 -
 
 ### Bug Fixes
+- main: suggest --os flag in unsupported OS error message to help users override ELF OS detection @devs6186 #2577
 - render: escape sample-controlled strings before passing to Rich to prevent MarkupError @devs6186 #2699
 - Fixed insecure deserialization vulnerability in YAML loading @0x1622 (#2770)
 - loader: gracefully handle ELF files with unsupported architectures kamranulhaq2002@gmail.com #2800

capa/helpers.py

@@ -327,6 +327,9 @@ def log_unsupported_os_error():
     logger.error(" ")
     logger.error(" capa currently only analyzes executables for some operating systems")
     logger.error(" (including Windows, Linux, and Android).")
+    logger.error(" ")
+    logger.error(" If you know the target OS, you can specify it explicitly, for example:")
+    logger.error("   capa --os linux <sample>")
     logger.error("-" * 80)