{"id":21782,"date":"2022-11-15T20:06:20","date_gmt":"2022-11-15T14:36:20","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/"},"modified":"2022-11-15T20:06:20","modified_gmt":"2022-11-15T14:36:20","slug":"solved-cs-strange-pointer-arithmetics-closed","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/","title":{"rendered":"[Solved] C’s strange pointer arithmetics [closed]"},"content":{"rendered":"

[ad_1]
\n<\/p>\n

\n
\n
<\/div>\n
\n
\n

As pointed out by others, this statement suffers from undefined behavior<\/a>:<\/p>\n

*(&x+5) += 7;\n<\/code><\/pre>\n
Memory address &x+5<\/code> is outside the bounds of variable x<\/code>. Writing to that address is a very bad idea.<\/em><\/p>\n
OP’s code sample exploits certain C compiler implementation details.
\nProbably educational<\/strong>; it can be used to demonstrate how hackers can exploit missing bounds checks to change the designed behavior of a program.<\/p>\n
It is interesting enough to investigate the actual behavior of this program.
\nI will assume the program has been compiled by GCC with default settings on a 64-bit Linux distro on Intel x64 processor architecture (i.e. little endian<\/a>).
\nPointers are 64 bits (8 bytes), int<\/code> is 32 bits (4 bytes).<\/p>\n
Variable x<\/code> is located on the stack. During execution of k()<\/code>, the stack looks like this:<\/p>\n
+--------+----------------------------------------------------+  TOP OF STACK\n| rsp+0  | unused; 16-byte alignment filler                   | <--- rsp\n+--------+----------------------------------------------------+\n| rsp+4  | x                                                  | <--- &x\n+--------+----------------------------------------------------+\n| rsp+8  | reserved; least significant 32 bits of stack guard |\n+--------+----------------------------------------------------+\n| rsp+12 | reserved; most significant 32 bits of stack guard  |\n+--------+----------------------------------------------------+\n| rsp+16 | least significant 32 bits of saved base pointer    | <--- rbp\n+--------+----------------------------------------------------+\n| rsp+20 | most significant 32 bits of saved base pointer     |\n+--------+----------------------------------------------------+\n| rsp+24 | least significant 32 bits of return address of k() | <--- &x+5\n+--------+----------------------------------------------------+\n| rsp+28 | most significant 32 bits of return address of k()  |\n+--------+----------------------------------------------------+\n| rsp+32 | start of stack frame of main()                     |\n+--------+----------------------------------------------------+\n<\/code><\/pre>\n
&x+5<\/code> is a memory address that is 20 bytes away from &x<\/code> (because sizeof(int)<\/code> is 4).
\nThat happens to be the location of the return address of k()<\/code>.
\n*(&x+5) += 7<\/code> will increase the return address by 7.
\nThat will have its effect when returning from k()<\/code> to main()<\/code>.
\nHere is objdump<\/code> output of main()<\/code>:<\/p>\n
void main(void)\n{\n  45:   55                      push   rbp\n  46:   48 89 e5                mov    rbp,rsp\n  49:   48 83 ec 10             sub    rsp,0x10\n        int y = 1;\n  4d:   c7 45 fc 01 00 00 00    mov    DWORD PTR [rbp-0x4],0x1\n        y = 13;\n  54:   c7 45 fc 0d 00 00 00    mov    DWORD PTR [rbp-0x4],0xd\n        k();\n  5b:   e8 00 00 00 00          call   60 <main+0x1b>\n        y = 2;\n  60:   c7 45 fc 02 00 00 00    mov    DWORD PTR [rbp-0x4],0x2\n        printf(\"y = %d\\n\", y);\n  67:   8b 45 fc                mov    eax,DWORD PTR [rbp-0x4]\n  6a:   89 c6                   mov    esi,eax\n  6c:   48 8d 3d 00 00 00 00    lea    rdi,[rip+0x0]        # 73 <main+0x2e>\n  73:   b8 00 00 00 00          mov    eax,0x0\n  78:   e8 00 00 00 00          call   7d <main+0x38>\n}\n  7d:   90                      nop\n  7e:   c9                      leave\n  7f:   c3                      ret\n<\/code><\/pre>\n
The program is supposed to return from k()<\/code> at offset 60<\/code> (the start of y = 2<\/code>), but instead, it will return at offset 67<\/code> (the start of printf(\"y = %d\\n\", y)<\/code>).<\/p>\n
In other words, after returning from k()<\/code>, the program will skip y = 2<\/code> and continue from printf(\"y = %d\\n\", y)<\/code>, printing y = 1<\/code>.<\/p>\n<\/p><\/div>\n
<\/div>\n<\/div>\n
            <\/span> <\/p><\/div>\n<\/div>\n
[ad_2]<\/p>\n
solved C’s strange pointer arithmetics [closed] <\/p>\n","protected":false},"excerpt":{"rendered":"
[ad_1] As pointed out by others, this statement suffers from undefined behavior: *(&x+5) += 7; Memory address &x+5 is outside the bounds of variable x. Writing to that address is a very bad idea. OP’s code sample exploits certain C compiler implementation details. Probably educational; it can be used to demonstrate how hackers can exploit … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[324,712],"class_list":["post-21782","post","type-post","status-publish","format-standard","hentry","category-solved","tag-c","tag-pointers"],"yoast_head":"\n[Solved] C's strange pointer arithmetics [closed] - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] C's strange pointer arithmetics [closed] - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] As pointed out by others, this statement suffers from undefined behavior: *(&x+5) += 7; Memory address &x+5 is outside the bounds of variable x. Writing to that address is a very bad idea. OP’s code sample exploits certain C compiler implementation details. Probably educational; it can be used to demonstrate how hackers can exploit ... Read more\" \/>\n<meta property=\"og:url\" content=\"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/\" \/>\n<meta property=\"og:site_name\" content=\"JassWeb\" \/>\n<meta property=\"article:published_time\" content=\"2022-11-15T14:36:20+00:00\" \/>\n<meta name=\"author\" content=\"Kirat\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Kirat\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"2 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/#article\",\"isPartOf\":{\"@id\":\"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/\"},\"author\":{\"name\":\"Kirat\",\"@id\":\"https:\/\/jassweb.com\/solved\/#\/schema\/person\/65c9c7b7958150c0dc8371fa35dd7c31\"},\"headline\":\"[Solved] C’s strange pointer arithmetics [closed]\",\"datePublished\":\"2022-11-15T14:36:20+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/\"},\"wordCount\":214,\"publisher\":{\"@id\":\"https:\/\/jassweb.com\/solved\/#organization\"},\"keywords\":[\"c++\",\"pointers\"],\"articleSection\":[\"Solved\"],\"inLanguage\":\"en-US\"},{\"@type\":\"WebPage\",\"@id\":\"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/\",\"url\":\"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/\",\"name\":\"[Solved] C's strange pointer arithmetics [closed] - JassWeb\",\"isPartOf\":{\"@id\":\"https:\/\/jassweb.com\/solved\/#website\"},\"datePublished\":\"2022-11-15T14:36:20+00:00\",\"breadcrumb\":{\"@id\":\"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/#breadcrumb\"},\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"ReadAction\",\"target\":[\"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/\"]}]},{\"@type\":\"BreadcrumbList\",\"@id\":\"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/#breadcrumb\",\"itemListElement\":[{\"@type\":\"ListItem\",\"position\":1,\"name\":\"Home\",\"item\":\"https:\/\/jassweb.com\/solved\/\"},{\"@type\":\"ListItem\",\"position\":2,\"name\":\"[Solved] C’s strange pointer arithmetics [closed]\"}]},{\"@type\":\"WebSite\",\"@id\":\"https:\/\/jassweb.com\/solved\/#website\",\"url\":\"https:\/\/jassweb.com\/solved\/\",\"name\":\"JassWeb\",\"description\":\"Build High-quality Websites\",\"publisher\":{\"@id\":\"https:\/\/jassweb.com\/solved\/#organization\"},\"potentialAction\":[{\"@type\":\"SearchAction\",\"target\":{\"@type\":\"EntryPoint\",\"urlTemplate\":\"https:\/\/jassweb.com\/solved\/?s={search_term_string}\"},\"query-input\":{\"@type\":\"PropertyValueSpecification\",\"valueRequired\":true,\"valueName\":\"search_term_string\"}}],\"inLanguage\":\"en-US\"},{\"@type\":\"Organization\",\"@id\":\"https:\/\/jassweb.com\/solved\/#organization\",\"name\":\"Jass Web\",\"url\":\"https:\/\/jassweb.com\/solved\/\",\"logo\":{\"@type\":\"ImageObject\",\"inLanguage\":\"en-US\",\"@id\":\"https:\/\/jassweb.com\/solved\/#\/schema\/logo\/image\/\",\"url\":\"https:\/\/jassweb.com\/wp-content\/uploads\/2021\/02\/jass-website-logo-1.png\",\"contentUrl\":\"https:\/\/jassweb.com\/wp-content\/uploads\/2021\/02\/jass-website-logo-1.png\",\"width\":693,\"height\":132,\"caption\":\"Jass Web\"},\"image\":{\"@id\":\"https:\/\/jassweb.com\/solved\/#\/schema\/logo\/image\/\"}},{\"@type\":\"Person\",\"@id\":\"https:\/\/jassweb.com\/solved\/#\/schema\/person\/65c9c7b7958150c0dc8371fa35dd7c31\",\"name\":\"Kirat\",\"image\":{\"@type\":\"ImageObject\",\"inLanguage\":\"en-US\",\"@id\":\"https:\/\/jassweb.com\/solved\/#\/schema\/person\/image\/\",\"url\":\"https:\/\/jassweb.com\/solved\/wp-content\/litespeed\/avatar\/1261af3c9451399fa1336d28b98ea3bb.jpg?ver=1775798750\",\"contentUrl\":\"https:\/\/jassweb.com\/solved\/wp-content\/litespeed\/avatar\/1261af3c9451399fa1336d28b98ea3bb.jpg?ver=1775798750\",\"caption\":\"Kirat\"},\"sameAs\":[\"http:\/\/jassweb.com\"],\"url\":\"https:\/\/jassweb.com\/solved\/author\/jaspritsinghghumangmail-com\/\"}]}<\/script>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"[Solved] C's strange pointer arithmetics [closed] - JassWeb","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/","og_locale":"en_US","og_type":"article","og_title":"[Solved] C's strange pointer arithmetics [closed] - JassWeb","og_description":"[ad_1] As pointed out by others, this statement suffers from undefined behavior: *(&x+5) += 7; Memory address &x+5 is outside the bounds of variable x. Writing to that address is a very bad idea. OP’s code sample exploits certain C compiler implementation details. Probably educational; it can be used to demonstrate how hackers can exploit ... Read more","og_url":"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/","og_site_name":"JassWeb","article_published_time":"2022-11-15T14:36:20+00:00","author":"Kirat","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Kirat","Est. reading time":"2 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/#article","isPartOf":{"@id":"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/"},"author":{"name":"Kirat","@id":"https:\/\/jassweb.com\/solved\/#\/schema\/person\/65c9c7b7958150c0dc8371fa35dd7c31"},"headline":"[Solved] C’s strange pointer arithmetics [closed]","datePublished":"2022-11-15T14:36:20+00:00","mainEntityOfPage":{"@id":"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/"},"wordCount":214,"publisher":{"@id":"https:\/\/jassweb.com\/solved\/#organization"},"keywords":["c++","pointers"],"articleSection":["Solved"],"inLanguage":"en-US"},{"@type":"WebPage","@id":"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/","url":"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/","name":"[Solved] C's strange pointer arithmetics [closed] - JassWeb","isPartOf":{"@id":"https:\/\/jassweb.com\/solved\/#website"},"datePublished":"2022-11-15T14:36:20+00:00","breadcrumb":{"@id":"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/#breadcrumb"},"inLanguage":"en-US","potentialAction":[{"@type":"ReadAction","target":["https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/"]}]},{"@type":"BreadcrumbList","@id":"https:\/\/jassweb.com\/solved\/solved-cs-strange-pointer-arithmetics-closed\/#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/jassweb.com\/solved\/"},{"@type":"ListItem","position":2,"name":"[Solved] C’s strange pointer arithmetics [closed]"}]},{"@type":"WebSite","@id":"https:\/\/jassweb.com\/solved\/#website","url":"https:\/\/jassweb.com\/solved\/","name":"JassWeb","description":"Build High-quality Websites","publisher":{"@id":"https:\/\/jassweb.com\/solved\/#organization"},"potentialAction":[{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https:\/\/jassweb.com\/solved\/?s={search_term_string}"},"query-input":{"@type":"PropertyValueSpecification","valueRequired":true,"valueName":"search_term_string"}}],"inLanguage":"en-US"},{"@type":"Organization","@id":"https:\/\/jassweb.com\/solved\/#organization","name":"Jass Web","url":"https:\/\/jassweb.com\/solved\/","logo":{"@type":"ImageObject","inLanguage":"en-US","@id":"https:\/\/jassweb.com\/solved\/#\/schema\/logo\/image\/","url":"https:\/\/jassweb.com\/wp-content\/uploads\/2021\/02\/jass-website-logo-1.png","contentUrl":"https:\/\/jassweb.com\/wp-content\/uploads\/2021\/02\/jass-website-logo-1.png","width":693,"height":132,"caption":"Jass Web"},"image":{"@id":"https:\/\/jassweb.com\/solved\/#\/schema\/logo\/image\/"}},{"@type":"Person","@id":"https:\/\/jassweb.com\/solved\/#\/schema\/person\/65c9c7b7958150c0dc8371fa35dd7c31","name":"Kirat","image":{"@type":"ImageObject","inLanguage":"en-US","@id":"https:\/\/jassweb.com\/solved\/#\/schema\/person\/image\/","url":"https:\/\/jassweb.com\/solved\/wp-content\/litespeed\/avatar\/1261af3c9451399fa1336d28b98ea3bb.jpg?ver=1775798750","contentUrl":"https:\/\/jassweb.com\/solved\/wp-content\/litespeed\/avatar\/1261af3c9451399fa1336d28b98ea3bb.jpg?ver=1775798750","caption":"Kirat"},"sameAs":["http:\/\/jassweb.com"],"url":"https:\/\/jassweb.com\/solved\/author\/jaspritsinghghumangmail-com\/"}]}},"_links":{"self":[{"href":"https:\/\/jassweb.com\/solved\/wp-json\/wp\/v2\/posts\/21782","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/jassweb.com\/solved\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/jassweb.com\/solved\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/jassweb.com\/solved\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/jassweb.com\/solved\/wp-json\/wp\/v2\/comments?post=21782"}],"version-history":[{"count":0,"href":"https:\/\/jassweb.com\/solved\/wp-json\/wp\/v2\/posts\/21782\/revisions"}],"wp:attachment":[{"href":"https:\/\/jassweb.com\/solved\/wp-json\/wp\/v2\/media?parent=21782"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/jassweb.com\/solved\/wp-json\/wp\/v2\/categories?post=21782"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/jassweb.com\/solved\/wp-json\/wp\/v2\/tags?post=21782"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}