Re: [PATCH 3/6] string: Introduce strtostr() for safe and performance string copies
From: David Laight <hidden>
Date: 2026-05-17 21:34:24
Also in:
linux-mm, lkml
On Sun, 17 May 2026 15:36:13 -0300 André Almeida [off-list ref] wrote:
quoted hunk ↗ jump to hunk
Some parts of the kernel uses memcpy() instead of strscpy() because they are performance sensitive and doesn't care about the return value of strscpy(). One such common case is to copy current->comm to a different buffer. As the command name is guaranteed to be NUL-terminated in the range of TASK_COMM_LEN, this is safe enough and doesn't create unterminated strings. However, in order to expand the size of current->comm, this expectation will be broken and those memcpy() could create such strings without trailing NUL byte. In order to support a fast and safe string copy, create strtostr(), to copy a NUL-terminated string to a new string buffer. If the destination buffer is bigger than the source, no pad is applied, but the string is NUL-terminated. If the destination buffer is smaller, the string is truncated. The last byte of the destination is always set to NUL for safety. Signed-off-by: André Almeida <andrealmeid@igalia.com> --- include/linux/coredump.h | 2 +- include/linux/string.h | 28 ++++++++++++++++++++++ include/linux/tracepoint.h | 4 ++-- include/trace/events/block.h | 10 ++++---- include/trace/events/coredump.h | 2 +- include/trace/events/f2fs.h | 4 ++-- include/trace/events/oom.h | 2 +- include/trace/events/osnoise.h | 2 +- include/trace/events/sched.h | 10 ++++---- include/trace/events/signal.h | 2 +- include/trace/events/task.h | 4 ++-- kernel/printk/nbcon.c | 2 +- kernel/printk/printk.c | 2 +- tools/bpf/bpftool/pids.c | 4 ++-- .../selftests/bpf/test_kmods/bpf_testmod-events.h | 2 +- 15 files changed, 54 insertions(+), 26 deletions(-)diff --git a/include/linux/coredump.h b/include/linux/coredump.h index 68861da4cf7c..b370ef69f673 100644 --- a/include/linux/coredump.h +++ b/include/linux/coredump.h@@ -54,7 +54,7 @@ extern void vfs_coredump(const kernel_siginfo_t *siginfo); do { \ char comm[TASK_COMM_LEN]; \ /* This will always be NUL terminated. */ \ - memcpy(comm, current->comm, sizeof(comm)); \ + strtostr(comm, current->comm); \ printk_ratelimited(Level "coredump: %d(%*pE): " Format "\n", \ task_tgid_vnr(current), (int)strlen(comm), comm, ##__VA_ARGS__); \ } while (0) \diff --git a/include/linux/string.h b/include/linux/string.h index b850bd91b3d8..ff1f59f4139c 100644 --- a/include/linux/string.h +++ b/include/linux/string.h@@ -445,6 +445,34 @@ void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count, memcpy(dest, src, strnlen(src, min(_src_len, _dest_len))); \ } while (0) +/** + * strtostr - Copy NUL-terminanted string to NUL-terminate string + * + * @dest: Pointer of destination string + * @src: Pointer to NUL-terminates string + * + * This is a replacement for strcpy() where the caller doesn't care about the + * return value and if the string is going to be truncated, albeit it needs + * to mark sure that it will be NUL-terminated. Intended for performance + * sensitive cases, such as tracing.
If you care about performance, and the destination isn't smaller (especially if the sizes are the same) then just use memcpy().
+ *
+ * If the destination is bigger than the source, no padding happens. It it's
+ * smaller the strings gets truncated.
+ *
+ * Both arguments needs to be arrays with lengths discoverable by the compiler.
+ */
+#define strtostr(dest, src) do { \
+ const size_t _dest_len = __must_be_cstr(dest) + \
+ ARRAY_SIZE(dest); \
+ const size_t _src_len = __must_be_cstr(src) + \
+ __builtin_object_size(src, 1); \
+ \
+ BUILD_BUG_ON(!__builtin_constant_p(_dest_len) || \
+ _dest_len == (size_t)-1); \
+ memcpy(dest, src, strnlen(src, min(_src_len, _dest_len))); \
+ dest[_dest_len - 1] = '\0'; \
+} while (0)That doesn't work (for all sorts of reasons). _dest_len can be the size of a pointer - no array check. You need to use __is_array() and sizeof () for both dest and src. You might have meant to check that _src_len is constant, not _dest_len. You must not leave the destination unterminated. __builtin_object_size(x->y,1) is also entirely useless! If you have a pointer to a structure that ends in an array then the object size of that array is SIZE_MAX (as if the array continues past the end of the structure). See https://godbolt.org/z/csenjfvxe (which I happened to prepare earlier today). __builtin_object_size(x->y,0) also seems to always return SIZE_MAX. You do get a sane answer for (x->y,3) on recent clang - but nowhere else. -- David