github.com/spotify/syslog-redirector-golang@v0.0.0-20140320174030-4859f03d829a/src/pkg/html/template/content.go (about)

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package template
     6  
     7  import (
     8  	"fmt"
     9  	"reflect"
    10  )
    11  
    12  // Strings of content from a trusted source.
    13  type (
    14  	// CSS encapsulates known safe content that matches any of:
    15  	//   1. The CSS3 stylesheet production, such as `p { color: purple }`.
    16  	//   2. The CSS3 rule production, such as `a[href=~"https:"].foo#bar`.
    17  	//   3. CSS3 declaration productions, such as `color: red; margin: 2px`.
    18  	//   4. The CSS3 value production, such as `rgba(0, 0, 255, 127)`.
    19  	// See http://www.w3.org/TR/css3-syntax/#style
    20  	CSS string
    21  
    22  	// HTML encapsulates a known safe HTML document fragment.
    23  	// It should not be used for HTML from a third-party, or HTML with
    24  	// unclosed tags or comments. The outputs of a sound HTML sanitizer
    25  	// and a template escaped by this package are fine for use with HTML.
    26  	HTML string
    27  
    28  	// HTMLAttr encapsulates an HTML attribute from a trusted source,
    29  	// for example, ` dir="ltr"`.
    30  	HTMLAttr string
    31  
    32  	// JS encapsulates a known safe EcmaScript5 Expression, for example,
    33  	// `(x + y * z())`.
    34  	// Template authors are responsible for ensuring that typed expressions
    35  	// do not break the intended precedence and that there is no
    36  	// statement/expression ambiguity as when passing an expression like
    37  	// "{ foo: bar() }\n['foo']()", which is both a valid Expression and a
    38  	// valid Program with a very different meaning.
    39  	JS string
    40  
    41  	// JSStr encapsulates a sequence of characters meant to be embedded
    42  	// between quotes in a JavaScript expression.
    43  	// The string must match a series of StringCharacters:
    44  	//   StringCharacter :: SourceCharacter but not `\` or LineTerminator
    45  	//                    | EscapeSequence
    46  	// Note that LineContinuations are not allowed.
    47  	// JSStr("foo\\nbar") is fine, but JSStr("foo\\\nbar") is not.
    48  	JSStr string
    49  
    50  	// URL encapsulates a known safe URL or URL substring (see RFC 3986).
    51  	// A URL like `javascript:checkThatFormNotEditedBeforeLeavingPage()`
    52  	// from a trusted source should go in the page, but by default dynamic
    53  	// `javascript:` URLs are filtered out since they are a frequently
    54  	// exploited injection vector.
    55  	URL string
    56  )
    57  
    58  type contentType uint8
    59  
    60  const (
    61  	contentTypePlain contentType = iota
    62  	contentTypeCSS
    63  	contentTypeHTML
    64  	contentTypeHTMLAttr
    65  	contentTypeJS
    66  	contentTypeJSStr
    67  	contentTypeURL
    68  	// contentTypeUnsafe is used in attr.go for values that affect how
    69  	// embedded content and network messages are formed, vetted,
    70  	// or interpreted; or which credentials network messages carry.
    71  	contentTypeUnsafe
    72  )
    73  
    74  // indirect returns the value, after dereferencing as many times
    75  // as necessary to reach the base type (or nil).
    76  func indirect(a interface{}) interface{} {
    77  	if a == nil {
    78  		return nil
    79  	}
    80  	if t := reflect.TypeOf(a); t.Kind() != reflect.Ptr {
    81  		// Avoid creating a reflect.Value if it's not a pointer.
    82  		return a
    83  	}
    84  	v := reflect.ValueOf(a)
    85  	for v.Kind() == reflect.Ptr && !v.IsNil() {
    86  		v = v.Elem()
    87  	}
    88  	return v.Interface()
    89  }
    90  
    91  var (
    92  	errorType       = reflect.TypeOf((*error)(nil)).Elem()
    93  	fmtStringerType = reflect.TypeOf((*fmt.Stringer)(nil)).Elem()
    94  )
    95  
    96  // indirectToStringerOrError returns the value, after dereferencing as many times
    97  // as necessary to reach the base type (or nil) or an implementation of fmt.Stringer
    98  // or error,
    99  func indirectToStringerOrError(a interface{}) interface{} {
   100  	if a == nil {
   101  		return nil
   102  	}
   103  	v := reflect.ValueOf(a)
   104  	for !v.Type().Implements(fmtStringerType) && !v.Type().Implements(errorType) && v.Kind() == reflect.Ptr && !v.IsNil() {
   105  		v = v.Elem()
   106  	}
   107  	return v.Interface()
   108  }
   109  
   110  // stringify converts its arguments to a string and the type of the content.
   111  // All pointers are dereferenced, as in the text/template package.
   112  func stringify(args ...interface{}) (string, contentType) {
   113  	if len(args) == 1 {
   114  		switch s := indirect(args[0]).(type) {
   115  		case string:
   116  			return s, contentTypePlain
   117  		case CSS:
   118  			return string(s), contentTypeCSS
   119  		case HTML:
   120  			return string(s), contentTypeHTML
   121  		case HTMLAttr:
   122  			return string(s), contentTypeHTMLAttr
   123  		case JS:
   124  			return string(s), contentTypeJS
   125  		case JSStr:
   126  			return string(s), contentTypeJSStr
   127  		case URL:
   128  			return string(s), contentTypeURL
   129  		}
   130  	}
   131  	for i, arg := range args {
   132  		args[i] = indirectToStringerOrError(arg)
   133  	}
   134  	return fmt.Sprint(args...), contentTypePlain
   135  }