[Lldb-commits] [PATCH] D73860: [lldb/StringPrinter] Avoid reading garbage in uninitialized strings

[Vedant Kumar via Phabricator via lldb-commits]

vsk marked an inline comment as done.
vsk added a comment.

Thanks everyone for the reviews!



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Comment at: lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/string/main.cpp:29
+    if (sizeof(std::string) == sizeof(garbage_string_sso))
+      memcpy((void *)&garbage1, &garbage_string_sso, sizeof(std::string));
+    if (sizeof(std::string) == sizeof(garbage_string_long))
----------------
vsk wrote:
> teemperor wrote:
> > teemperor wrote:
> > > shafik wrote:
> > > > vsk wrote:
> > > > > shafik wrote:
> > > > > > While I get what you are doing here, we know he structure of libc++ SSO implementation and we are manually building a corrupt one, this is fragile to changes in the implementation. 
> > > > > > 
> > > > > > I don't have an immediate suggestion for an alternative approach but if we stick with this we should stick a big comment explaining this, perhaps laying out the assumptions of the internal layout we are assuming and maybe some sanity checks maybe using `offsetof` to verify fields exist and are where we expect them to be.
> > > > > I don't see how this is fragile. The structure of libc++'s SSO implementation is ABI, and is unlikely to change (esp. not in a way that turns either one of the garbage strings into a valid string). I've left comments explaining what's wrong with both of the garbage strings, but can leave a pointer to https://joellaity.com/2020/01/31/string.html for more info?
> > > > Sure, that note would be fine.
> > > Can you instead do a `#if _LIBCPP_ABI_VERSION == 1` and have the #else as an #error that this test needs updating. We don't support any other libc++ ABI beside 1 in LLDB but if we ever do then this should not silently pass.
> > Actually the #if *and* a static_assert comparing the size would be best IMHO.
> Sure, but the size check is not primarily about the ABI. The garbage examples presuppose 64-bit pointer & size types, which is not true on some watches.
I'm not sure how to write a static assert that isn't a little brittle. Maybe `static_assert(sizeof(void *) != 8 || sizeof(std::string) == 24, "unknown std::string layout")`?


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  https://reviews.llvm.org/D73860/new/

https://reviews.llvm.org/D73860