[v2,03/28] lib/string.c: implement stpcpy

Commit Message

Sami Tolvanen Sept. 3, 2020, 8:30 p.m. UTC

From: Nick Desaulniers <[e-mail]>

LLVM implemented a recent "libcall optimization" that lowers calls to
`sprintf(dest, "%s", str)` where the return value is used to
`stpcpy(dest, str) - dest`. This generally avoids the machinery involved
in parsing format strings.  `stpcpy` is just like `strcpy` except it
returns the pointer to the new tail of `dest`.  This optimization was
introduced into clang-12.

Implement this so that we don't observe linkage failures due to missing
symbol definitions for `stpcpy`.

Similar to last year's fire drill with:
commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")

The kernel is somewhere between a "freestanding" environment (no full libc)
and "hosted" environment (many symbols from libc exist with the same
type, function signature, and semantics).

As H. Peter Anvin notes, there's not really a great way to inform the
compiler that you're targeting a freestanding environment but would like
to opt-in to some libcall optimizations (see pr/47280 below), rather than
opt-out.

Arvind notes, -fno-builtin-* behaves slightly differently between GCC
and Clang, and Clang is missing many __builtin_* definitions, which I
consider a bug in Clang and am working on fixing.

Masahiro summarizes the subtle distinction between compilers justly:
  To prevent transformation from foo() into bar(), there are two ways in
  Clang to do that; -fno-builtin-foo, and -fno-builtin-bar.  There is
  only one in GCC; -fno-buitin-foo.

(Any difference in that behavior in Clang is likely a bug from a missing
__builtin_* definition.)

Masahiro also notes:
  We want to disable optimization from foo() to bar(),
  but we may still benefit from the optimization from
  foo() into something else. If GCC implements the same transform, we
  would run into a problem because it is not -fno-builtin-bar, but
  -fno-builtin-foo that disables that optimization.

  In this regard, -fno-builtin-foo would be more future-proof than
  -fno-built-bar, but -fno-builtin-foo is still potentially overkill. We
  may want to prevent calls from foo() being optimized into calls to
  bar(), but we still may want other optimization on calls to foo().

It seems that compilers today don't quite provide the fine grain control
over which libcall optimizations pseudo-freestanding environments would
prefer.

Finally, Kees notes that this interface is unsafe, so we should not
encourage its use.  As such, I've removed the declaration from any
header, but it still needs to be exported to avoid linkage errors in
modules.

Reported-by: Sami Tolvanen <[e-mail]>
Suggested-by: Andy Lavr <[e-mail]>
Suggested-by: Arvind Sankar <[e-mail]>
Suggested-by: Joe Perches <[e-mail]>
Suggested-by: Masahiro Yamada <[e-mail]>
Suggested-by: Rasmus Villemoes <[e-mail]>
Signed-off-by: Nick Desaulniers <[e-mail]>
Cc: [e-mail]
Link: https://bugs.llvm.org/show_bug.cgi?id=47162
Link: https://bugs.llvm.org/show_bug.cgi?id=47280
Link: https://github.com/ClangBuiltLinux/linux/issues/1126
Link: https://man7.org/linux/man-pages/man3/stpcpy.3.html
Link: https://pubs.opengroup.org/onlinepubs/9699919799/functions/stpcpy.html
Link: https://reviews.llvm.org/D85963
---
 lib/string.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

Comments

Kees Cook Sept. 3, 2020, 9:47 p.m. UTC | #1

On Thu, Sep 03, 2020 at 01:30:28PM -0700, Sami Tolvanen wrote:
> From: Nick Desaulniers <[e-mail]>
> 
> LLVM implemented a recent "libcall optimization" that lowers calls to
> `sprintf(dest, "%s", str)` where the return value is used to
> `stpcpy(dest, str) - dest`. This generally avoids the machinery involved
> in parsing format strings.  `stpcpy` is just like `strcpy` except it
> returns the pointer to the new tail of `dest`.  This optimization was
> introduced into clang-12.
> 
> Implement this so that we don't observe linkage failures due to missing
> symbol definitions for `stpcpy`.
> 
> Similar to last year's fire drill with:
> commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")
> 
> The kernel is somewhere between a "freestanding" environment (no full libc)
> and "hosted" environment (many symbols from libc exist with the same
> type, function signature, and semantics).
> 
> As H. Peter Anvin notes, there's not really a great way to inform the
> compiler that you're targeting a freestanding environment but would like
> to opt-in to some libcall optimizations (see pr/47280 below), rather than
> opt-out.
> 
> Arvind notes, -fno-builtin-* behaves slightly differently between GCC
> and Clang, and Clang is missing many __builtin_* definitions, which I
> consider a bug in Clang and am working on fixing.
> 
> Masahiro summarizes the subtle distinction between compilers justly:
>   To prevent transformation from foo() into bar(), there are two ways in
>   Clang to do that; -fno-builtin-foo, and -fno-builtin-bar.  There is
>   only one in GCC; -fno-buitin-foo.
> 
> (Any difference in that behavior in Clang is likely a bug from a missing
> __builtin_* definition.)
> 
> Masahiro also notes:
>   We want to disable optimization from foo() to bar(),
>   but we may still benefit from the optimization from
>   foo() into something else. If GCC implements the same transform, we
>   would run into a problem because it is not -fno-builtin-bar, but
>   -fno-builtin-foo that disables that optimization.
> 
>   In this regard, -fno-builtin-foo would be more future-proof than
>   -fno-built-bar, but -fno-builtin-foo is still potentially overkill. We
>   may want to prevent calls from foo() being optimized into calls to
>   bar(), but we still may want other optimization on calls to foo().
> 
> It seems that compilers today don't quite provide the fine grain control
> over which libcall optimizations pseudo-freestanding environments would
> prefer.
> 
> Finally, Kees notes that this interface is unsafe, so we should not
> encourage its use.  As such, I've removed the declaration from any
> header, but it still needs to be exported to avoid linkage errors in
> modules.
> 
> Reported-by: Sami Tolvanen <[e-mail]>
> Suggested-by: Andy Lavr <[e-mail]>
> Suggested-by: Arvind Sankar <[e-mail]>
> Suggested-by: Joe Perches <[e-mail]>
> Suggested-by: Masahiro Yamada <[e-mail]>
> Suggested-by: Rasmus Villemoes <[e-mail]>
> Signed-off-by: Nick Desaulniers <[e-mail]>

As you mentioned, this is in -next already (via -mm). I think I sent a
tag for this before, but maybe akpm missed it, so for good measure:

Reviewed-by: Kees Cook <[e-mail]>

Patch

diff --git a/lib/string.c b/lib/string.c
index 6012c385fb31..6bd0cf0fb009 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -272,6 +272,30 @@  ssize_t strscpy_pad(char *dest, const char *src, size_t count)
 }
 EXPORT_SYMBOL(strscpy_pad);
 
+/**
+ * stpcpy - copy a string from src to dest returning a pointer to the new end
+ *          of dest, including src's %NUL-terminator. May overrun dest.
+ * @dest: pointer to end of string being copied into. Must be large enough
+ *        to receive copy.
+ * @src: pointer to the beginning of string being copied from. Must not overlap
+ *       dest.
+ *
+ * stpcpy differs from strcpy in a key way: the return value is the new
+ * %NUL-terminated character. (for strcpy, the return value is a pointer to
+ * src. This interface is considered unsafe as it doesn't perform bounds
+ * checking of the inputs. As such it's not recommended for usage. Instead,
+ * its definition is provided in case the compiler lowers other libcalls to
+ * stpcpy.
+ */
+char *stpcpy(char *__restrict__ dest, const char *__restrict__ src);
+char *stpcpy(char *__restrict__ dest, const char *__restrict__ src)
+{
+	while ((*dest++ = *src++) != '\0')
+		/* nothing */;
+	return --dest;
+}
+EXPORT_SYMBOL(stpcpy);
+
 #ifndef __HAVE_ARCH_STRCAT
 /**
  * strcat - Append one %NUL-terminated string to another