Why doesn't Linux support mmap by path?

The name of the pictureThe name of the pictureThe name of the pictureClash Royale CLAN TAG#URR8PPP












2















The mmap syscall needs a fd as parameter, but when you close that fd, the mmap is still alive in the process's memory address space.



Therefore keeping an mmap doesn't need an opened fd, so why dose Linux only support creating an mmap of a file using a fd, but not a file-name-path? Wouldn't it be nice if we can have a mmapat syscall just like openat and execveat?



If mmap creates an extra reference to that file, why can't we have a mmapat which atomically creates such an reference at the first time without take an fd of the process then release it later.



Is there any historical or security reason for not having such syscall on Linux kernel?










share|improve this question



















  • 1





    The *_at differ from their non-at counterparts by being able to resolve relative path wrt to an open fd to a directory instead of the cwd, not by using a path instead of a file descriptor. As to why your mmap_path() is not implemented in the kernel, the keyword from your question is "atomically". Good luck doing it atomically.

    – mosvy
    Feb 4 at 4:59












  • @mosvy But mmap never support path. While other functions support path either *at or non-at version.

    – 炸鱼薯条德里克
    Feb 4 at 5:06











  • That's why I was suggesting you to rename it mmap_path() ;-)

    – mosvy
    Feb 4 at 5:08















2















The mmap syscall needs a fd as parameter, but when you close that fd, the mmap is still alive in the process's memory address space.



Therefore keeping an mmap doesn't need an opened fd, so why dose Linux only support creating an mmap of a file using a fd, but not a file-name-path? Wouldn't it be nice if we can have a mmapat syscall just like openat and execveat?



If mmap creates an extra reference to that file, why can't we have a mmapat which atomically creates such an reference at the first time without take an fd of the process then release it later.



Is there any historical or security reason for not having such syscall on Linux kernel?










share|improve this question



















  • 1





    The *_at differ from their non-at counterparts by being able to resolve relative path wrt to an open fd to a directory instead of the cwd, not by using a path instead of a file descriptor. As to why your mmap_path() is not implemented in the kernel, the keyword from your question is "atomically". Good luck doing it atomically.

    – mosvy
    Feb 4 at 4:59












  • @mosvy But mmap never support path. While other functions support path either *at or non-at version.

    – 炸鱼薯条德里克
    Feb 4 at 5:06











  • That's why I was suggesting you to rename it mmap_path() ;-)

    – mosvy
    Feb 4 at 5:08













2












2








2


0






The mmap syscall needs a fd as parameter, but when you close that fd, the mmap is still alive in the process's memory address space.



Therefore keeping an mmap doesn't need an opened fd, so why dose Linux only support creating an mmap of a file using a fd, but not a file-name-path? Wouldn't it be nice if we can have a mmapat syscall just like openat and execveat?



If mmap creates an extra reference to that file, why can't we have a mmapat which atomically creates such an reference at the first time without take an fd of the process then release it later.



Is there any historical or security reason for not having such syscall on Linux kernel?










share|improve this question
















The mmap syscall needs a fd as parameter, but when you close that fd, the mmap is still alive in the process's memory address space.



Therefore keeping an mmap doesn't need an opened fd, so why dose Linux only support creating an mmap of a file using a fd, but not a file-name-path? Wouldn't it be nice if we can have a mmapat syscall just like openat and execveat?



If mmap creates an extra reference to that file, why can't we have a mmapat which atomically creates such an reference at the first time without take an fd of the process then release it later.



Is there any historical or security reason for not having such syscall on Linux kernel?







linux-kernel






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Feb 12 at 10:26









ctrl-alt-delor

11.6k42159




11.6k42159










asked Feb 2 at 1:32









炸鱼薯条德里克炸鱼薯条德里克

510214




510214







  • 1





    The *_at differ from their non-at counterparts by being able to resolve relative path wrt to an open fd to a directory instead of the cwd, not by using a path instead of a file descriptor. As to why your mmap_path() is not implemented in the kernel, the keyword from your question is "atomically". Good luck doing it atomically.

    – mosvy
    Feb 4 at 4:59












  • @mosvy But mmap never support path. While other functions support path either *at or non-at version.

    – 炸鱼薯条德里克
    Feb 4 at 5:06











  • That's why I was suggesting you to rename it mmap_path() ;-)

    – mosvy
    Feb 4 at 5:08












  • 1





    The *_at differ from their non-at counterparts by being able to resolve relative path wrt to an open fd to a directory instead of the cwd, not by using a path instead of a file descriptor. As to why your mmap_path() is not implemented in the kernel, the keyword from your question is "atomically". Good luck doing it atomically.

    – mosvy
    Feb 4 at 4:59












  • @mosvy But mmap never support path. While other functions support path either *at or non-at version.

    – 炸鱼薯条德里克
    Feb 4 at 5:06











  • That's why I was suggesting you to rename it mmap_path() ;-)

    – mosvy
    Feb 4 at 5:08







1




1





The *_at differ from their non-at counterparts by being able to resolve relative path wrt to an open fd to a directory instead of the cwd, not by using a path instead of a file descriptor. As to why your mmap_path() is not implemented in the kernel, the keyword from your question is "atomically". Good luck doing it atomically.

– mosvy
Feb 4 at 4:59






The *_at differ from their non-at counterparts by being able to resolve relative path wrt to an open fd to a directory instead of the cwd, not by using a path instead of a file descriptor. As to why your mmap_path() is not implemented in the kernel, the keyword from your question is "atomically". Good luck doing it atomically.

– mosvy
Feb 4 at 4:59














@mosvy But mmap never support path. While other functions support path either *at or non-at version.

– 炸鱼薯条德里克
Feb 4 at 5:06





@mosvy But mmap never support path. While other functions support path either *at or non-at version.

– 炸鱼薯条德里克
Feb 4 at 5:06













That's why I was suggesting you to rename it mmap_path() ;-)

– mosvy
Feb 4 at 5:08





That's why I was suggesting you to rename it mmap_path() ;-)

– mosvy
Feb 4 at 5:08










1 Answer
1






active

oldest

votes


















4














mmap(2) is basically a read(2) to a buffer that the kernel is going to write to directly, rather than reading to a kernel buffer and then copying to the provided buffer.



There's no mmapat(2) for the same reason there's no readat(2).



The file descriptor isn't needed any more because




The mmap() function adds an extra reference to the file associated with the file descriptor fildes which is not removed by a subsequent close() on that file descriptor. This reference is removed when there are no more mappings to the file.




This is per the OpenGroup's website. They're the people who mange the POSIX standard, if I'm not mistaken, so they should know.






share|improve this answer

























  • You can't write to a mmaped file after all fd of it are closed even though the mmap is PROT_WRITE|MAP_SHARED ?

    – 炸鱼薯条德里克
    Feb 2 at 3:24












  • I didn't say whether or not you can write to an mmapped file after the fd is closed. I said if you can write to the file through the closed fd then it's magic.

    – Ed Grimm
    Feb 2 at 3:32











  • And just found the reference I was looking for, and yeah, it's magic. Specifically, implicit dup(2) magic. I should learn to date-check references before accepting them. Google returned two links, and I guessed the first would be the better link because Google. Sigh.

    – Ed Grimm
    Feb 2 at 3:39












  • Yeah, but if it creates an extra reference, why can't we have mmapat which create such extra reference at the first time?

    – 炸鱼薯条德里克
    Feb 2 at 3:48






  • 1





    mmap is basically an alternate implementation of read. You can if you want write the wrapper function you're asking for and use it yourself, but you'll have about as much luck getting a standard library to incorporate it as you would having them incorporate readat.

    – Ed Grimm
    Feb 2 at 3:51










Your Answer








StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "106"
;
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function()
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled)
StackExchange.using("snippets", function()
createEditor();
);

else
createEditor();

);

function createEditor()
StackExchange.prepareEditor(
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: false,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: null,
bindNavPrevention: true,
postfix: "",
imageUploader:
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
,
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
);



);













draft saved

draft discarded


















StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f498236%2fwhy-doesnt-linux-support-mmap-by-path%23new-answer', 'question_page');

);

Post as a guest















Required, but never shown

























1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









4














mmap(2) is basically a read(2) to a buffer that the kernel is going to write to directly, rather than reading to a kernel buffer and then copying to the provided buffer.



There's no mmapat(2) for the same reason there's no readat(2).



The file descriptor isn't needed any more because




The mmap() function adds an extra reference to the file associated with the file descriptor fildes which is not removed by a subsequent close() on that file descriptor. This reference is removed when there are no more mappings to the file.




This is per the OpenGroup's website. They're the people who mange the POSIX standard, if I'm not mistaken, so they should know.






share|improve this answer

























  • You can't write to a mmaped file after all fd of it are closed even though the mmap is PROT_WRITE|MAP_SHARED ?

    – 炸鱼薯条德里克
    Feb 2 at 3:24












  • I didn't say whether or not you can write to an mmapped file after the fd is closed. I said if you can write to the file through the closed fd then it's magic.

    – Ed Grimm
    Feb 2 at 3:32











  • And just found the reference I was looking for, and yeah, it's magic. Specifically, implicit dup(2) magic. I should learn to date-check references before accepting them. Google returned two links, and I guessed the first would be the better link because Google. Sigh.

    – Ed Grimm
    Feb 2 at 3:39












  • Yeah, but if it creates an extra reference, why can't we have mmapat which create such extra reference at the first time?

    – 炸鱼薯条德里克
    Feb 2 at 3:48






  • 1





    mmap is basically an alternate implementation of read. You can if you want write the wrapper function you're asking for and use it yourself, but you'll have about as much luck getting a standard library to incorporate it as you would having them incorporate readat.

    – Ed Grimm
    Feb 2 at 3:51















4














mmap(2) is basically a read(2) to a buffer that the kernel is going to write to directly, rather than reading to a kernel buffer and then copying to the provided buffer.



There's no mmapat(2) for the same reason there's no readat(2).



The file descriptor isn't needed any more because




The mmap() function adds an extra reference to the file associated with the file descriptor fildes which is not removed by a subsequent close() on that file descriptor. This reference is removed when there are no more mappings to the file.




This is per the OpenGroup's website. They're the people who mange the POSIX standard, if I'm not mistaken, so they should know.






share|improve this answer

























  • You can't write to a mmaped file after all fd of it are closed even though the mmap is PROT_WRITE|MAP_SHARED ?

    – 炸鱼薯条德里克
    Feb 2 at 3:24












  • I didn't say whether or not you can write to an mmapped file after the fd is closed. I said if you can write to the file through the closed fd then it's magic.

    – Ed Grimm
    Feb 2 at 3:32











  • And just found the reference I was looking for, and yeah, it's magic. Specifically, implicit dup(2) magic. I should learn to date-check references before accepting them. Google returned two links, and I guessed the first would be the better link because Google. Sigh.

    – Ed Grimm
    Feb 2 at 3:39












  • Yeah, but if it creates an extra reference, why can't we have mmapat which create such extra reference at the first time?

    – 炸鱼薯条德里克
    Feb 2 at 3:48






  • 1





    mmap is basically an alternate implementation of read. You can if you want write the wrapper function you're asking for and use it yourself, but you'll have about as much luck getting a standard library to incorporate it as you would having them incorporate readat.

    – Ed Grimm
    Feb 2 at 3:51













4












4








4







mmap(2) is basically a read(2) to a buffer that the kernel is going to write to directly, rather than reading to a kernel buffer and then copying to the provided buffer.



There's no mmapat(2) for the same reason there's no readat(2).



The file descriptor isn't needed any more because




The mmap() function adds an extra reference to the file associated with the file descriptor fildes which is not removed by a subsequent close() on that file descriptor. This reference is removed when there are no more mappings to the file.




This is per the OpenGroup's website. They're the people who mange the POSIX standard, if I'm not mistaken, so they should know.






share|improve this answer















mmap(2) is basically a read(2) to a buffer that the kernel is going to write to directly, rather than reading to a kernel buffer and then copying to the provided buffer.



There's no mmapat(2) for the same reason there's no readat(2).



The file descriptor isn't needed any more because




The mmap() function adds an extra reference to the file associated with the file descriptor fildes which is not removed by a subsequent close() on that file descriptor. This reference is removed when there are no more mappings to the file.




This is per the OpenGroup's website. They're the people who mange the POSIX standard, if I'm not mistaken, so they should know.







share|improve this answer














share|improve this answer



share|improve this answer








edited Feb 2 at 3:38

























answered Feb 2 at 3:01









Ed GrimmEd Grimm

3737




3737












  • You can't write to a mmaped file after all fd of it are closed even though the mmap is PROT_WRITE|MAP_SHARED ?

    – 炸鱼薯条德里克
    Feb 2 at 3:24












  • I didn't say whether or not you can write to an mmapped file after the fd is closed. I said if you can write to the file through the closed fd then it's magic.

    – Ed Grimm
    Feb 2 at 3:32











  • And just found the reference I was looking for, and yeah, it's magic. Specifically, implicit dup(2) magic. I should learn to date-check references before accepting them. Google returned two links, and I guessed the first would be the better link because Google. Sigh.

    – Ed Grimm
    Feb 2 at 3:39












  • Yeah, but if it creates an extra reference, why can't we have mmapat which create such extra reference at the first time?

    – 炸鱼薯条德里克
    Feb 2 at 3:48






  • 1





    mmap is basically an alternate implementation of read. You can if you want write the wrapper function you're asking for and use it yourself, but you'll have about as much luck getting a standard library to incorporate it as you would having them incorporate readat.

    – Ed Grimm
    Feb 2 at 3:51

















  • You can't write to a mmaped file after all fd of it are closed even though the mmap is PROT_WRITE|MAP_SHARED ?

    – 炸鱼薯条德里克
    Feb 2 at 3:24












  • I didn't say whether or not you can write to an mmapped file after the fd is closed. I said if you can write to the file through the closed fd then it's magic.

    – Ed Grimm
    Feb 2 at 3:32











  • And just found the reference I was looking for, and yeah, it's magic. Specifically, implicit dup(2) magic. I should learn to date-check references before accepting them. Google returned two links, and I guessed the first would be the better link because Google. Sigh.

    – Ed Grimm
    Feb 2 at 3:39












  • Yeah, but if it creates an extra reference, why can't we have mmapat which create such extra reference at the first time?

    – 炸鱼薯条德里克
    Feb 2 at 3:48






  • 1





    mmap is basically an alternate implementation of read. You can if you want write the wrapper function you're asking for and use it yourself, but you'll have about as much luck getting a standard library to incorporate it as you would having them incorporate readat.

    – Ed Grimm
    Feb 2 at 3:51
















You can't write to a mmaped file after all fd of it are closed even though the mmap is PROT_WRITE|MAP_SHARED ?

– 炸鱼薯条德里克
Feb 2 at 3:24






You can't write to a mmaped file after all fd of it are closed even though the mmap is PROT_WRITE|MAP_SHARED ?

– 炸鱼薯条德里克
Feb 2 at 3:24














I didn't say whether or not you can write to an mmapped file after the fd is closed. I said if you can write to the file through the closed fd then it's magic.

– Ed Grimm
Feb 2 at 3:32





I didn't say whether or not you can write to an mmapped file after the fd is closed. I said if you can write to the file through the closed fd then it's magic.

– Ed Grimm
Feb 2 at 3:32













And just found the reference I was looking for, and yeah, it's magic. Specifically, implicit dup(2) magic. I should learn to date-check references before accepting them. Google returned two links, and I guessed the first would be the better link because Google. Sigh.

– Ed Grimm
Feb 2 at 3:39






And just found the reference I was looking for, and yeah, it's magic. Specifically, implicit dup(2) magic. I should learn to date-check references before accepting them. Google returned two links, and I guessed the first would be the better link because Google. Sigh.

– Ed Grimm
Feb 2 at 3:39














Yeah, but if it creates an extra reference, why can't we have mmapat which create such extra reference at the first time?

– 炸鱼薯条德里克
Feb 2 at 3:48





Yeah, but if it creates an extra reference, why can't we have mmapat which create such extra reference at the first time?

– 炸鱼薯条德里克
Feb 2 at 3:48




1




1





mmap is basically an alternate implementation of read. You can if you want write the wrapper function you're asking for and use it yourself, but you'll have about as much luck getting a standard library to incorporate it as you would having them incorporate readat.

– Ed Grimm
Feb 2 at 3:51





mmap is basically an alternate implementation of read. You can if you want write the wrapper function you're asking for and use it yourself, but you'll have about as much luck getting a standard library to incorporate it as you would having them incorporate readat.

– Ed Grimm
Feb 2 at 3:51

















draft saved

draft discarded
















































Thanks for contributing an answer to Unix & Linux Stack Exchange!


  • Please be sure to answer the question. Provide details and share your research!

But avoid


  • Asking for help, clarification, or responding to other answers.

  • Making statements based on opinion; back them up with references or personal experience.

To learn more, see our tips on writing great answers.




draft saved


draft discarded














StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f498236%2fwhy-doesnt-linux-support-mmap-by-path%23new-answer', 'question_page');

);

Post as a guest















Required, but never shown





















































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown

































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown






Popular posts from this blog

How to check contact read email or not when send email to Individual?

Displaying single band from multi-band raster using QGIS

How many registers does an x86_64 CPU actually have?