Re: [PATCH 4/5] strbuf_readlink(): support link targets that exceed PATH_MAX

[Patrick Steinhardt <ps@pks.im>]

On Fri, Dec 19, 2025 at 09:50:15AM +0100, Johannes Schindelin wrote:
> Hi Patrick,
> 
> On Wed, 17 Dec 2025, Patrick Steinhardt wrote:
> 
> > On Tue, Dec 16, 2025 at 03:33:48PM +0000, Karsten Blees via GitGitGadget wrote:
> > > diff --git a/strbuf.c b/strbuf.c
> > > index 44a8f6a554..fa4e30f112 100644
> > > --- a/strbuf.c
> > > +++ b/strbuf.c
> > > @@ -566,8 +566,6 @@ ssize_t strbuf_write(struct strbuf *sb, FILE *f)
> > >  	return sb->len ? fwrite(sb->buf, 1, sb->len, f) : 0;
> > >  }
> > >  
> > > -#define STRBUF_MAXLINK (2*PATH_MAX)
> > > -
> > >  int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint)
> > >  {
> > >  	size_t oldalloc = sb->alloc;
> > > @@ -575,7 +573,7 @@ int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint)
> > >  	if (hint < 32)
> > >  		hint = 32;
> > >  
> > > -	while (hint < STRBUF_MAXLINK) {
> > > +	for (;;) {
> > >  		ssize_t len;
> > >  
> > >  		strbuf_grow(sb, hint + 1);
> > 
> > This makes me wonder whether we have a better way to figure out the
> > actual size of the buffer that we ultimately need to allocate. But
> > reading through readlink(3p) doesn't indicate anything, and I'm not sure
> > whether we can always rely on lstat(3p) to return the correct size for
> > symlink contents on all platforms.
> > 
> > One thing that _is_ noted though is that calling the function with a
> > buffer size larger than SSIZE_MAX is implementation-defined. It does
> > make me a bit uneasy in that light to grow indefinitely.
> > 
> > Which makes me wonder whether Windows has a limit for the symlink
> > contents that we could enforce in theory so that we can reasonably turn
> > this into a bounded loop again?
> 
> https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation
> suggests that the maximum permissible target path should be 32,768. But
> that's not _quite_ correct, as
> `../t/../Documentation/RelNotes/../../README.md` is a perfectly valid (if
> awkward) symlink target.
> 
> Still, I would say that 32,768 would make for a fine (still insanely high,
> but not so high as to allow malicious symlinks to cause memory problems)
> limit.
> 
> Sound good?
> Johannes

Sounds good to me, thanks!

Patrick