[PATCH v4] lib/string.c: implement stpcpy

[PATCH v4] lib/string.c: implement stpcpy

[Nick Desaulniers]

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 <samitolvanen@google.com>
Suggested-by: Andy Lavr <andy.lavr@gmail.com>
Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
Suggested-by: Joe Perches <joe@perches.com>
Suggested-by: Kees Cook <keescook@chromium.org>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
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
---
Changes V4:
* Roll up Kees' comment fixup from
  https://lore.kernel.org/lkml/202009060302.4574D8D0E0@keescook/#t.
* Keep Nathan's tested by tag.
* Add Kees' reviewed by tag from
  https://lore.kernel.org/lkml/202009031446.3865FE82B@keescook/.

Changes V3:
* Drop Sami's Tested by tag; newer patch.
* Add EXPORT_SYMBOL as per Andy.
* Rewrite commit message, rewrote part of what Masahiro said to be
  generic in terms of foo() and bar().
* Prefer %NUL-terminated to NULL terminated. NUL is the ASCII character
  '\0', as per Arvind and Rasmus.

Changes V2:
* Added Sami's Tested by; though the patch changed implementation, the
  missing symbol at link time was the problem Sami was observing.
* Fix __restrict -> __restrict__ typo as per Joe.
* Drop note about restrict from commit message as per Arvind.
* Fix NULL -> NUL as per Arvind; NUL is ASCII '\0'. TIL
* Fix off by one error as per Arvind; I had another off by one error in
  my test program that was masking this.
 lib/string.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/lib/string.c b/lib/string.c
index 6012c385fb31..b6b8847218b5 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 a pointer
+ * to the new %NUL-terminating character in @dest. (For strcpy, the return
+ * value is a pointer to the start of @dest. 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
-- 
2.28.0.618.gf4bc123cb7-goog

Re: [PATCH v4] lib/string.c: implement stpcpy

[Nick Desaulniers]

On Mon, Sep 14, 2020 at 9:10 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> 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 <samitolvanen@google.com>
> Suggested-by: Andy Lavr <andy.lavr@gmail.com>
> Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
> Suggested-by: Joe Perches <joe@perches.com>
> Suggested-by: Kees Cook <keescook@chromium.org>
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> Tested-by: Nathan Chancellor <natechancellor@gmail.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Cc: stable@vger.kernel.org
> 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
> ---
> Changes V4:
> * Roll up Kees' comment fixup from
>   https://lore.kernel.org/lkml/202009060302.4574D8D0E0@keescook/#t.
> * Keep Nathan's tested by tag.
> * Add Kees' reviewed by tag from
>   https://lore.kernel.org/lkml/202009031446.3865FE82B@keescook/.
>
> Changes V3:
> * Drop Sami's Tested by tag; newer patch.
> * Add EXPORT_SYMBOL as per Andy.
> * Rewrite commit message, rewrote part of what Masahiro said to be
>   generic in terms of foo() and bar().
> * Prefer %NUL-terminated to NULL terminated. NUL is the ASCII character
>   '\0', as per Arvind and Rasmus.
>
> Changes V2:
> * Added Sami's Tested by; though the patch changed implementation, the
>   missing symbol at link time was the problem Sami was observing.
> * Fix __restrict -> __restrict__ typo as per Joe.
> * Drop note about restrict from commit message as per Arvind.
> * Fix NULL -> NUL as per Arvind; NUL is ASCII '\0'. TIL
> * Fix off by one error as per Arvind; I had another off by one error in
>   my test program that was masking this.
>  lib/string.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/lib/string.c b/lib/string.c
> index 6012c385fb31..b6b8847218b5 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 a pointer
> + * to the new %NUL-terminating character in @dest. (For strcpy, the return
> + * value is a pointer to the start of @dest. This interface is considered

And, I forgot the stupid close parens...I will resend a v5...

> + * 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
> --
> 2.28.0.618.gf4bc123cb7-goog
>


-- 
Thanks,
~Nick Desaulniers

[PATCH v5] lib/string.c: implement stpcpy

[Nick Desaulniers]

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 <samitolvanen@google.com>
Suggested-by: Andy Lavr <andy.lavr@gmail.com>
Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
Suggested-by: Joe Perches <joe@perches.com>
Suggested-by: Kees Cook <keescook@chromium.org>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Cc: stable@vger.kernel.org
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
---
Changes V5:
* fix missing parens in comment.

Changes V4:
* Roll up Kees' comment fixup from
  https://lore.kernel.org/lkml/202009060302.4574D8D0E0@keescook/#t.
* Keep Nathan's tested by tag.
* Add Kees' reviewed by tag from
  https://lore.kernel.org/lkml/202009031446.3865FE82B@keescook/.

Changes V3:
* Drop Sami's Tested by tag; newer patch.
* Add EXPORT_SYMBOL as per Andy.
* Rewrite commit message, rewrote part of what Masahiro said to be
  generic in terms of foo() and bar().
* Prefer %NUL-terminated to NULL terminated. NUL is the ASCII character
  '\0', as per Arvind and Rasmus.

Changes V2:
* Added Sami's Tested by; though the patch changed implementation, the
  missing symbol at link time was the problem Sami was observing.
* Fix __restrict -> __restrict__ typo as per Joe.
* Drop note about restrict from commit message as per Arvind.
* Fix NULL -> NUL as per Arvind; NUL is ASCII '\0'. TIL
* Fix off by one error as per Arvind; I had another off by one error in
  my test program that was masking this.

 lib/string.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/lib/string.c b/lib/string.c
index 6012c385fb31..4288e0158d47 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 a pointer
+ * to the new %NUL-terminating character in @dest. (For strcpy, the return
+ * value is a pointer to the start of @dest). 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
-- 
2.28.0.618.gf4bc123cb7-goog

Re: [PATCH v5] lib/string.c: implement stpcpy

[Nathan Chancellor]

On Mon, Sep 14, 2020 at 09:16:43AM -0700, Nick Desaulniers wrote:
> 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 <samitolvanen@google.com>
> Suggested-by: Andy Lavr <andy.lavr@gmail.com>
> Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
> Suggested-by: Joe Perches <joe@perches.com>
> Suggested-by: Kees Cook <keescook@chromium.org>
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> Tested-by: Nathan Chancellor <natechancellor@gmail.com>

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>

It would be nice to get this into mainline sooner rather than later so
that it can start filtering into the stable trees. ToT LLVM builds have
been broken for a month now.

> Cc: stable@vger.kernel.org
> 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
> ---
> Changes V5:
> * fix missing parens in comment.
> 
> Changes V4:
> * Roll up Kees' comment fixup from
>   https://lore.kernel.org/lkml/202009060302.4574D8D0E0@keescook/#t.
> * Keep Nathan's tested by tag.
> * Add Kees' reviewed by tag from
>   https://lore.kernel.org/lkml/202009031446.3865FE82B@keescook/.

For the record, I don't see Kees' review tag on this.

> 
> Changes V3:
> * Drop Sami's Tested by tag; newer patch.
> * Add EXPORT_SYMBOL as per Andy.
> * Rewrite commit message, rewrote part of what Masahiro said to be
>   generic in terms of foo() and bar().
> * Prefer %NUL-terminated to NULL terminated. NUL is the ASCII character
>   '\0', as per Arvind and Rasmus.
> 
> Changes V2:
> * Added Sami's Tested by; though the patch changed implementation, the
>   missing symbol at link time was the problem Sami was observing.
> * Fix __restrict -> __restrict__ typo as per Joe.
> * Drop note about restrict from commit message as per Arvind.
> * Fix NULL -> NUL as per Arvind; NUL is ASCII '\0'. TIL
> * Fix off by one error as per Arvind; I had another off by one error in
>   my test program that was masking this.
> 
>  lib/string.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/lib/string.c b/lib/string.c
> index 6012c385fb31..4288e0158d47 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 a pointer
> + * to the new %NUL-terminating character in @dest. (For strcpy, the return
> + * value is a pointer to the start of @dest). 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
> -- 
> 2.28.0.618.gf4bc123cb7-goog
> 

Re: [PATCH v5] lib/string.c: implement stpcpy

[Joe Perches]

On Mon, 2020-09-14 at 21:22 -0700, Nathan Chancellor wrote:
> It would be nice to get this into mainline sooner rather than later so
> that it can start filtering into the stable trees. ToT LLVM builds have
> been broken for a month now.

People that build stable trees with new compilers
unsupported at the time the of the base version
release are just asking for trouble.

Re: [PATCH v5] lib/string.c: implement stpcpy

[Nick Desaulniers]

On Mon, Sep 14, 2020 at 9:22 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> It would be nice to get this into mainline sooner rather than later so
> that it can start filtering into the stable trees. ToT LLVM builds have
> been broken for a month now.

Hi Andrew, I appreciate your help getting this submitted to mainline.

Noting that other folks are hitting this frequently:
https://lore.kernel.org/lkml/CAKwvOdkh=bZE6uY8zk_QePq5B3fY1ue9VjEguJ_cQi4CtZ4xgw@mail.gmail.com/

CrOS folks are already shipping v3 downstream since this is blocking
the release of their toolchain.

(Also, I appreciate folks' thoughts on the comments in the patch, but
please stop delaying this patch from hitting mainline.  You can
rewrite the commit message to whatever you want, delete it for all I
care, please for god's sake please unbreak the build first).
-- 
Thanks,
~Nick Desaulniers

Re: [PATCH v5] lib/string.c: implement stpcpy

[Nick Desaulniers]

On Mon, Sep 14, 2020 at 9:28 PM Joe Perches <joe@perches.com> wrote:
>
> On Mon, 2020-09-14 at 21:22 -0700, Nathan Chancellor wrote:
> > It would be nice to get this into mainline sooner rather than later so
> > that it can start filtering into the stable trees. ToT LLVM builds have
> > been broken for a month now.
>
> People that build stable trees with new compilers
> unsupported at the time the of the base version
> release are just asking for trouble.

It is asymmetry that we have a minimum supported version of a
toolchain, but no maximum supported version of a toolchain for a given
branch.  I think that's a good thing; imagine if you were stuck on an
old compiler for a stable branch.  No thanks.  I guess we just like to
live dangerously? :P

Also, GKH has voiced support for newer toolchains for older kernel
releases before.  Related to this issue, in fact.
https://lore.kernel.org/lkml/20200818072531.GC9254@kroah.com/
--
Thanks,
~Nick Desaulniers