Fix: pad strings that are modified concurrently with tracing

[lttng-ust.git] / libringbuffer / backend.h

diff --git a/libringbuffer/backend.h b/libringbuffer/backend.h
index feefc7a3229f1314cf6813b1f05178a0cc6176e8..53a97140bf33bfd70c21f6b7939c1f48251cb20f 100644 (file)
--- a/libringbuffer/backend.h
+++ b/libringbuffer/backend.h
@@ -105,6 +105,96 @@ void lib_ring_buffer_write(const struct lttng_ust_lib_ring_buffer_config *config
        ctx->buf_offset += len;
 }
 
+/*
+ * Copy up to @len string bytes from @src to @dest. Stop whenever a NULL
+ * terminating character is found in @src. Returns the number of bytes
+ * copied. Does *not* terminate @dest with NULL terminating character.
+ */
+static inline
+size_t lib_ring_buffer_do_strcpy(const struct lttng_ust_lib_ring_buffer_config *config,
+               char *dest, const char *src, size_t len)
+{
+       size_t count;
+
+       for (count = 0; count < len; count++) {
+               char c;
+
+               /*
+                * Only read source character once, in case it is
+                * modified concurrently.
+                */
+               c = CMM_LOAD_SHARED(src[count]);
+               if (!c)
+                       break;
+               lib_ring_buffer_do_copy(config, &dest[count], &c, 1);
+       }
+       return count;
+}
+
+/**
+ * lib_ring_buffer_strcpy - write string data to a buffer backend
+ * @config : ring buffer instance configuration
+ * @ctx: ring buffer context. (input arguments only)
+ * @src : source pointer to copy from
+ * @len : length of data to copy
+ * @pad : character to use for padding
+ *
+ * This function copies @len - 1 bytes of string data from a source
+ * pointer to a buffer backend, followed by a terminating '\0'
+ * character, at the current context offset. This is more or less a
+ * buffer backend-specific strncpy() operation. If a terminating '\0'
+ * character is found in @src before @len - 1 characters are copied, pad
+ * the buffer with @pad characters (e.g. '#').
+ */
+static inline
+void lib_ring_buffer_strcpy(const struct lttng_ust_lib_ring_buffer_config *config,
+                          struct lttng_ust_lib_ring_buffer_ctx *ctx,
+                          const char *src, size_t len, int pad)
+{
+       struct lttng_ust_lib_ring_buffer_backend *bufb = &ctx->buf->backend;
+       struct channel_backend *chanb = &ctx->chan->backend;
+       struct lttng_ust_shm_handle *handle = ctx->handle;
+       size_t sbidx, count;
+       size_t offset = ctx->buf_offset;
+       struct lttng_ust_lib_ring_buffer_backend_pages_shmp *rpages;
+       unsigned long sb_bindex, id;
+
+       if (caa_unlikely(!len))
+               return;
+       offset &= chanb->buf_size - 1;
+       sbidx = offset >> chanb->subbuf_size_order;
+       id = shmp_index(handle, bufb->buf_wsb, sbidx)->id;
+       sb_bindex = subbuffer_id_get_index(config, id);
+       rpages = shmp_index(handle, bufb->array, sb_bindex);
+       CHAN_WARN_ON(ctx->chan,
+                    config->mode == RING_BUFFER_OVERWRITE
+                    && subbuffer_id_is_noref(config, id));
+       /*
+        * Underlying layer should never ask for writes across
+        * subbuffers.
+        */
+       CHAN_WARN_ON(chanb, offset >= chanb->buf_size);
+       count = lib_ring_buffer_do_strcpy(config,
+                       shmp_index(handle, shmp(handle, rpages->shmp)->p,
+                               offset & (chanb->subbuf_size - 1)),
+                       src, len - 1);
+       offset += count;
+       /* Padding */
+       if (caa_unlikely(count < len - 1)) {
+               size_t pad_len = len - 1 - count;
+
+               lib_ring_buffer_do_memset(shmp_index(handle, shmp(handle, rpages->shmp)->p,
+                               offset & (chanb->subbuf_size - 1)),
+                       pad, pad_len);
+               offset += pad_len;
+       }
+       /* Final '\0' */
+       lib_ring_buffer_do_memset(shmp_index(handle, shmp(handle, rpages->shmp)->p,
+                               offset & (chanb->subbuf_size - 1)),
+                       '\0', 1);
+       ctx->buf_offset += len;
+}
+
 /*
  * This accessor counts the number of unread records in a buffer.
  * It only provides a consistent value if no reads not writes are performed